I would think it was strange.
This is all hypothetical, and I’m not married, so I don’t know how to answer. But if it were any one of my last four “major” girlfriends, and the person in trouble was one of their actual friends that I’ve met, I’d be totally OK with it.
1) If you are the spouse, what is your initial reaction? Does your reaction change with more thought?
My initial reaction is, if this guy needs my wife’s help in his time of crisis so badly, and she wants to go, then more power to her. I’ll drive her to the airport and take care of the boys all weekend. But I’ll admit that the more I think about it, the more troubled I am. I trust my wife implicitly, but who knows just how needy this other guy is? Who knows what might happen while he’s on his emotional rollercoaster and she’s doing her best to be caring and sympathetic? It’s a situation that could be ripe for trouble. I take my marriage vows very seriously; I hope and expect my wife will, too, and I have no reason to think that she ever hasn’t. But people do uncharacteristic things, things they might very well regret later, things that can badly hurt those whom they love, while under stress and emotional turmoil.
2) Does it make any difference what sex the married person and spouse are?
Probably not. I expect my wife would have pretty much the same thought process.
3) Does it make any difference, positively or negatively, that Friend is paying for the trip?
No difference at all.
If this comes off as insensitive as hell, then I’m just a lout and always will be; what the hell is with the “preferred sex” business? If we’re talking male-female here, let’s talk male-female, because that sexual dynamic is completely different from the dynamic among three homosexuals, two of whom just happen to be married.
Having said that, I can only speak as a heterosexual male. If my boyhood almost-sister-friend called from the opposite coast (I actually don’t live on a coast, but if I did) and wanted me to come hold her hand while she worked through some stuff, I’d make plans for the wife and I to go. Razorette and I come as a package – my friends are her friends. It’s cost me some friendships over the years (including that of my real-life almost-sister-friend) but yes, she’s that important to me.
You seem to think that I have made these rules for my husband, so that I will make sure he doesn’t stray, and vice versa. In fact, he makes those rules for himself and I make them for myself, because that’s what we feel is appropriate for a married couple. If there’s any lack of trust, it’s my husband who does not trust himself to remain completely untempted, and the same for me. I know that I’m only a regular woman, not some superuntemptablewoman, and though I love my husband dearly and would never want to cheat on him—well, others have said the same and fallen into workplace affairs or whatever, and I’m not stronger or better than them. So I choose to draw the line for myself at not spending time alone with other men. My husband gets that time and focus of energy, because he’s my priority.
If we’re talking about lunch, I wouldn’t go out to lunch alone with another guy; I’d ask other friends along too and make it a group thing. Why not? If I wanted to visit an old guy friend, it would be a couples or group visit.
I’m not sure it’s right to put these tests of trust on to people. People make mistakes, none of us are invulnerable. So saying “I trust you completely and therefore you can go participate in naked wrestling with Playboy Playmates, because I know they won’t even turn you on!” --it isn’t realistic or fair. So, yes, I trust my husband not to want to cheat on me and to behave according to his principles. I don’t expect him to be Superman; he’s not. There’s no such person. But since I know that he wouldn’t put himself in the position of having to be Superman in the first place, I don’t worry about it.
But is that really the question? When I read the OP, I don’t see question as “Would your spouse want to do this?” The question is “Would you be okay with your spouse doing this?”
So an answer like “It’s not that I don’t trust him but he wouldn’t want to do this” isn’t answering the question.
I’ll say the same thing I always say in threads like these:
I’d like to believe that the reason my wife doesn’t have sex with other men is that she chooses not to do it, not simply that she doesn’t have or avoids the opportunity.
Given that, I’d be fine with it if it were her going, and don’t expect she’d have much of a problem if it were me.
I thought about it and think that I would be a little nervous about my husband going to help such a friend, but I don’t feel it would be a rational nervousness considering his character.
But if she cooked him biscuits and gravy and fried cornbread, he might never come home. I think my husband is a food whore. 
Well that makes more sense then. Your original post though stated
“And then there’s a lot of talk about trust on this thread, implying that only couples who don’t trust each other would have such a rule. But it’s really not about trust. I trust my husband, he’s one of the best men I know–that’s why I married him. But we also know that we are human beings, and human beings are never 100% mistake-proof and invulnerable to temptation. That’s just how it is.”
which seemed to imply that you trust him yet didn’t trust him. What you are saying is that it is trusting yourself to not be tempted. And I do agree with that–we each are the soul judge of what we might do. So I think I understand what you are saying and good for you and your husband that you have the rules that work for you. My wife and I have different rules but the same basic outcome.
I think the extreme examples of playboy bunnies notwithstanding, people of opposite sex can be friends and I do trust myself to not be tempted. I just had coffee with an old friend of mine, she is also a good friend of my wife. It was coffee and I enjoyed the time I spent with her. But I would not want to have a group coffee–I wanted to talk to her and catch up with what is happening with her and her husband. In fact we decided at coffee to all four of us to get together. Sound great to me!
I love my wife and there is and would never be any temptation for me with our friend. She is attractive–but the thing is I love my wife and my wife is even more attractive to me. I don’t see it as tempting myself, I see it as having coffee with an old friend.
This is how I’d feel, too. He’s never cheated on me, I don’t believe he ever would, but yet I’d be uncomfortable for him to go to visit a female friend somewhere without me. And I know very well that even though I’ve never given him reason not to trust me, he’d be very unhappy if I even asked to go, because that would mean that at that moment I considered that friend’s needs to be more important than his and spending time with him. And I don’t think he’d be okay with that.
The more I think about it, the more offensive I find it that anybody would even make this request. Calling an opposite sex friend you haven’t seen for years and asking him/her to just drop everything out the blue and ditch his/her family and travel cross country to come and cater to your personal drama? Entitled much?
And it does have the distinctive ring of a Hail Mary booty call.
The parties have been in regular (weekly, and sometimes daily) communication over the years. They had not seen each other in person for a few years. Don’t know if that makes any difference.
I have a sneaking suspicion that you are the “friend.” Am I warm?
I am involved in this situation, but no, I am not Friend.
Come on—spill the beans already 
Ok, then I don’t feel as bad about insulting the friend. 
So you’re one of the spouses? Why not just spill it. You’ve been kind of defensive, so that makes me think you’re the invitee and that your wife isn’t thrilled. Am I getting warmer now?
Probably impossible to keep some of my bias out of this, so I’m not surprised that I come off as defensive.
Yes, I’m the invitee, but no, my wife has no problem with the situation. Our feelings are very similar to the way storyteller0910 put it. Not that I believe it would work for all relationships, but it’s the attitude that my wife and I have about each other, ourselves, and our marriage. My wife has been aware for some time about the issues the friend has been going through, and when I told her what was being proposed, she was supportive immediately.
We looked over our calendar and realized that there wasn’t any way for us to both get out there, but that there was a weekend coming up very soon where she would be tied up with activities that didn’t necessarily involve me, and suggested that would be the perfect time for me to go.
I want to emphasize that I don’t think the people who have posted in here are wrong for saying this would not work for them. It is part of the reason I didn’t come out from the beginning and say that this was me. In fact, I had wanted to post this thread as a hypothetical, but my wife, in her infinite wisdom, suggested that I would have come off as dishonest. But I just wanted a view of how other people see things like this because my general experience in life has been that my marriage is in a very small minority when it comes to how my wife and I view our relationships with other people.
I’m kind of glad you stated this as a “preferred sex” question, instead of nailing down the genders, because I’m rather surprised by the results. I would have to answer “absolutely no problem with the trip” and I’m pretty sure my spouse would, too.
I’ve been out as a lesbian for 25 years or so, my partner a similar amount of time. We have been active in the lesbian communities we’ve lived in, and those have been across the country. Our friends, surprise, surprise, tend to be lesbians.
If we took this “no travelling to see friends of the perferred sex” thing to heart, we’d have no friends at all!
Questions like this, though, make me wonder if all straight people aren’t a little bit crazy… you really have to vet your friends for correct gender?
Bingo.
I don’t see how sex enters the picture at all. Most of my friends are girls and, except for travelling across the country, situations like this happen quite often for me. My wife has no problem with me helping out my friends, for whatever reason, who happen to be girls. Most of her friends are girls too, but if she had close guy friends I don’t see why that would make a difference to me. She wouldn’t want me to wrestle naked with Playboy playmates, but she wouldn’t want me to wrestle naked with Chippendale dancers either. Basically, whatever rules you have for each other, that’s great – but the sex of the people involved shouldn’t really matter.
Not married, never have been married, but… I think there’s a huge difference depending on the character of the friend and the character of your relationship to the friend.
There are some people who are drama queens, who get worked up about every last litte thing, who are constantly getting in and out of crises, who might get drunk and come on to you.
There are other people who are none of the above, and who would only make a clearly-potentially-awkward request if it was truly an incredibly emotionally devestatingly serious time for them and they had no one else to turn to.
I have a female friend who I’m very close to who lives in Minnesota. Her father died suddenly a few years back, and she moved back to Minnesota to live with and support (and be supported by) her mother. If her mother suddenly tragically died, I would immediately want to fly out there to be there for her, even though she has friends and relatives there already. If I were married, I’d presumably still immediately want to fly out there. (Note that this woman and I shared a (two-bedroom) apartment for years without any sex ever occuring, which would presumably be reassuring to my hypothetical wife.)
I’m a bit puzzled by people saying “he’s MY security blanket/rock, get your own”. Is your spouse’s ability-to-comfort-another-human-being a limited resource? Can a spouse have only one real meaningful deep human connection?