Yes, I would.
Yet another addition to the list of things that “don’t count.”
“Eatin’ ain’t cheatin’”
“What happens in Vegas stays in Vegas”
“It was just Spring Break”
“We’d just had that big argument”
“It was someone I met on a business trip who I’ll never see again”
“They meant nothing to me. You’re the mother/father of our children. That’s what counts.”
“You did something that ‘doesn’t count,’ so I did too.”
"It was 1920 and I was George Babbit at an Elks convention: "I love my wife, but oh, you kid!'l
Basically a non-monogamous species trying to uphold an artificial standard of monogamy. Or, to be cynical, a not especially nice species trying to be unrealisticaly nice and only making things worse.
I disagree. The two things are not the same.
While I concur that girl-on-girl that hasn’t been consented to by the husband is cheating regardless, a lot of guys really aren’t going to consider it to be nearly as serious a cheat as with a guy.
I think there are good reasons for that, even without going back to the bad old male view of women as more or less property, and one’s fellow men being the only ones who are really a threat to steal or taint your property. Although no matter how enlightened a guy is, there’s some of that kicking around in the back of our skulls because all that doesn’t get undone in a generation or two. And pretending it isn’t there doesn’t help you not be that part of yourself.
But from a less prehistoric view, my wife and I have a zone of intimacy, if you will. (Not a great phrase, but couldn’t come up with a better one.) We are familiar with and comfortable with each other’s bodies. But at the same time, I certainly have the occasional “if I were single and she were willing…” moment when thinking about another woman. And I was intimate with other women before I met my wife, and if she predeceased me, I might well be intimate with other women afterwards. There’s a part of me that’s fundamentally okay with letting another woman into my zone of intimacy.
But there’s no significant part of me that’s okay with letting a man into my zone of intimacy. Don’t want to kiss one, don’t want to touch one, don’t want to have to try to be comfortable with another man’s nakedness wandering around my personal spaces. And part of my personal space (and hers as well, but that’s not the issue here) is the zone of intimacy that my wife and I have.
A man invading that zone of intimacy is, quite simply, anathema. But with a woman, there’s no inherent bar. Her having sex with a woman (not that she’s got the least interest) without my consent would be cheating, sure, but if she asked my consent in advance, it would be more, “well, how’s this going to work? What effect is it going to have on our life and our marriage?” While if she asked me if she could have sex with some guy, the answer would be “only if you’re asking for a divorce.”
Her cheating with a guy is going without my consent into territory that would be anathema regardless. Her cheating with a woman is going without my consent into territory that would otherwise have been merely problematic. So it would bother me considerably less.
I certainly wouldn’t say every guy feels this way, but it’s clear that a lot of guys react on something like this level.
I don’t see how acknowledging that somehow implies that gay and lesbian relationships ‘don’t count.’
“We were on a break!”
Let’s just say girl-girl is a big turn on for me. Not just “big”, HUGE!
That said, I would not want to share my partner, nor would I want her to want to share me.
I think it’s cheating, without question.
Of course it’s cheating, unless there is some kind of agreement allowing it.
Women’s sexuality has often been marginalized. Basically, to a lot of people, it’s the dick that counts.
I thought about this more, and I would just like to add that I would be JUST as upset if I found out my SO was with another woman as I would be another man. I hate to admit this, but I might even be more insecure about her being with another woman than another man. I don’t know why.
I wonder if some women would be more upset with their boyfriends/husbands if they found out they were with another man.
I’m OK with my GF getting it on with other girls if she’s OK with me getting it on with other… girls. 
But seriously, matters of degree of openness or exclusivity are something you really should square away clearly early in the relationship.
So yes, it is adulterous, and cheating, if the other partner/spouse was under the impression this was a committed-exclusive pairing.
Like someone else mentioned, a “crossover-sex doesn’t count” approach would be the wrong way to look at it: it does so “count”, the question is how would the other person feel, and what would s/he do, about it. Committed relationship = care about the other person’s feelings, remember? Others above have mentioned why instinctively, the “gut” reaction may be worse for a het cheating than for a same-sex cheating. The thing is, however, that how does someone deal with the situation depends strictly* on each individual* and IMO there’s no appeal to “common” wisdom as to how s/he should.
It is entirely valid for someone to say (I’ll borrow RTF’s terminology for convenience’s sake) “well, no, monogamy to me means NOBODY is allowed into the mutual Zone Of Intimacy, period”, and no reason to act incredulous (as reported in the OP) that there’s disagreement about “exceptions”. The OP report of all her friends agreeing? Irrelevant. The friends’ incredulity? Silly. The feelings of each of the two parties in the allegedly committed-exclusive pair are what counts.
Without any other arrangement discussed beforehand, my default assumption would be, yeah, of course it’s cheating. It is somewhat surprising to me to think otherwise.
First, I’m going to point out that everything you said is entirely from a man’s point of view (and, as you acknowledged, not a universal one).
Second, I will once again say that if someone is in a relationship with a prior arrangement regarding these matters their arrangement takes precedent in the relationship.
From my viewpoint (hetero monogamous female) I don’t do casual sex. If there isn’t already some sort of emotional connection there it’s not happening for me. As it happens, my spouse is the same. That doesn’t mean we don’t look at other people without lust - we certainly do appreciate good looking humans of our preferred gender - but neither of us is the sort of person who has ever engaged in casual hook ups. That’s really the sort of male partner I’ve always gravitated towards.
So, in my relationship, if ANY cheating has occurred it’s already on the level of an emotional relationship, which is the sort of thing that easily leads to divorce.
I do understand, intellectually, that there are people capable of casual sex that’s just the physical rubbing of body parts but emotionally I myself have never been in that space. So, on a gut level, I’m not going to believe it’s just “nothing” or a “fling” or a “one time thing” that “doesn’t mean anything”.
That’s why, to me, it IS the same thing. I’m not worried about my partner getting pregnant (obviously, he’s a guy after all), I’m worried about him forming an attachment to someone else and leaving me. Whether that other person is male or female is a bit irrelevant if the result is he’s not in my life anymore.
As a gay guy I find the opinion to be baffling that for many straight men in the world and in this thread that their wives or girlfriends cheating on them with another woman wouldn’t be that big of a deal. It honestly seems like a bit of a cuckold fantasy of sorts.
I can tell you right here and now that whether my boyfriend cheated on me with a girl or a guy, I’d be equally upset.
Although honestly, if you’ve married a closeted lesbian, your relationship is already doomed. The sooner she figures herself out and moves on, the happier both parties will be in the long run.
for some people emotional intimacy and sexual intimacy are separate.
it might be saying having sexual fun as long as there is no emotional intimacy is OK. this assuming a mostly hetero value on the sexuality scale. this would be emotional monogamy.
something similar might be the case with swingers who don’t allow kissing or snuggling because that is too emotional intimate (or would lead to such).
relationships need to be customized to the people involved. if it is safe, sane and consensual then what ever they agree to do is their relationship.
Having sex with more than one partner is not monogamy. If that’s your thing, you’ve ventured into a limited form of non-monogamy, and there’s nothing wrong with that, but call it what it is. It may or may not be “cheating” depending on the terms of your relationship. In my (polyamorous) relationships, cheating is defined by lying and sneaking around, not simple non-monogamy, which is usually open and above board. There is no universal rule in relationships, except to communicate clearly, honestly and often with your partner about your feelings and expectations.
Sorry, but I’m really not following the connection between my post and your response.
For a lot of straight men it’s still all about them, their dicks, and the paternity of their offspring. Girl-girl doesn’t count because it’s not involving them, not involving their dicks, and doesn’t affect the paternity of their children and therefore isn’t really sex.
Sorry if that comes off a bit harsh.
Well, sure. With a whole lot of piggish hetero men, though, (myself included) there’s an implicit agreement allowing it. I just wish my wife would agree to it. 
No shit! The OP’s question is fundamentally about how straight men feel about it.
If person X does something to person Y that person Y is generally recognized to have the right to be upset about, then it’s really up to person Y to decide whether or not they’re going to take offense.
You sound like Ani:
i must have blown a fuse or something
cause it was so dark in my mind
she came up to me with the sweetest face
and she was holding a light of some kind
and i still think of you as my boyfriend
i don’t think this is the end of the world
but i think maybe you should follow my example
and go meet yourself a really nice girl
As a hetero male, I can tell you the girl-thing is totally different from the cuckold fantasy. They’re, like, from different sides of the universe. They’re not in the same zone.