Mark’s proposal makes sense, but if Fay is uncomfortable with the idea for any reason, Mark shouldn’t push it.
It’s fine that he suggested it. It’s fine that he told her his wife was okay with it. But if this is somewhere Fay chooses to draw the line, abide by her wishes.
Re-reading the OP, all we’re told is that “Wendy rarely goes” with them. It’s assuming facts not in evidence to then say that Wendy is choosing to stay home alone instead. Maybe during their dinner/movie time, Wendy is busy at a volunteer gig, or providing in-home care to her elderly relatives, or engaging her time in some other way.
Shit, maybe Wendy just hates watching movies and is thrilled “to outsource this to another woman” so she doesn’t have to sit through the flicks herself.
Could be. I had a period where anytime my friend’s wife wanted to see a play or something she’d call me. My friend’s attitude was “Better you than me, man”.
Nothing hinky going on, just company for an outing.
I’m surprised no-one seems to think this reeks to high heaven; I read it as Fay not wanting ruin her and M’s special thing - befriending Wendy might put her in Mark’s true ‘friendzone’, while her current position is more ambiguous (at least in her mind).
Fay needs a boyfriend.
The whole situation seems weird to me. I agree that Fay’s boundaries are strange. She’s ok going out alone with a married man, but not ok being home alone with him. What’s the difference? She thinks Wendy will suddenly become jealous? If she’s so concerned about that, she should probably just end the relationship with Mark. She doesn’t trust herself (or Mark) to behave appropriately in a private setting? Then she should end her relationship with Mark.
I don’t get it, maybe these kinds of relationships can work. The one similar situation I have personally observed did not end happily.
I didn’t want to be the one to say it, but really … yes… this.
Editing to add this, which is also answered by my comments below:
Yes, and also as witnesses. Plus theaters and restaurants tend not to have rooms one can retire to where the door can be closed and which are outfitted with beds.
If for some reason there were a suspicion of hanky panky raised, one can make reasonable refutation for an environment that’s a public place (even if dark) with lots of observers around – and this would tend to prevent any suspicion in the first place. For the environment of a private home, alone together, there is plenty of room for suspicion and only one’s own protests to refute the charge. It can make a difference.
I can see the difference. Being with someone in their home is a lot more intimate than hanging out in a public place. It’s just a different vibe, and it can shift the dynamic between people. I mean, even among my girlfriends, there are very few that I have had in my home/been to their home, especially one-on-one. That’s a close friends thing to do.
I don’t think it’s a matter of “if I am alone in your house with you, we’ll have sex”. I think it’s more like “if we evolve into the kind of friends who hang out in each other’s houses, we may well become the sort of friends that represent a threat to a marriage”. I think that’s a reasonable boundary.
I’m not a CPA, but I agree with every word of this. There was a time in my life I probably would’ve felt differently, but now, knowing what I know, and the experiences I’ve had, I would err on the side of caution.
If Fay does have some romantic feelings for Mark, I imagine the last thing she’d want to do is see him in HER house.
Or maybe it’s perfectly innocent.
Aiming to fill the bingo card up early wih the easy spaces? 
Since we don’t know these people and we’re all just batting about ideas like a bunch of kittens with a wadded up ball of paper…
It has been my humble experience that for the most part, if something looks suspicious, people like to think the worst of it. I think it allows them to forgive the nastiness in themselves to think bad things about other people; if they don’t know the truth, they think up something exciting! Let the speculations begin!! :rolleyes:
Who are the witnesses? If Fay is seen going into Mark and Wendy’s house while Wendy is at work, WHAT will the neighbors think?! … Mark might as well have a scarlet letter. Wendy is cast in the role of the poor clueless wife. Fay is “the other woman.” (Anyone remember Mrs. Gladys Kravitz?)
Add to that that **some **people’s recollection of the passage of time is hazy at best.
(Background: my boyfriend works out of town.) About 6 months ago, I was sitting in the living room, and a friend of my boyfriend came over. They were visiting in the kitchen. (Friend is slightly obnoxious, so I was being “antisocial.” ) I overheard the friend say, “Hey, a week ago, I saw Why Child at the library!” First, I found it a little odd that he needed to report my activities to my boyfriend. As I mulled it over, I realized it had been two weeks since I’d been to the library.
So what if some well meaning (nosy, vindictive) neighbor says to Wendy, “I saw Mark bring home a woman (described as Fay,) on (the wrong day…)”
That could cause problems.
Or maybe Fay’s just paranoid. :rolleyes: 
I think Fay needs to acknowledge that she’s implying that either she or Mark might have feelings for the other.
That said, I have boundaries like this that might seem odd. I find it nearly impossible not to be attracted to female friends that I’m close enough to, so I’ve always drawn these types of lines to avoid temptation. The alternative would be no close female friends, which kinda sucks.
If that’s what Fay thinks, it’s fine. But she should have given an excuse rather than allude to something improper-- which is both awkward and presumptuous.
Fay should wait at Mark and Wendy’s house. While there she should scrunch up a pair of her panties and shove them down between the couch cushions.
When Wendy eventually confronts herr, Fay can scream, “See! I told ya!”
What’s right or wrong aside, now that Fay has made a “thing” about this being a “thing”, I think it’s highly likely that at some time in the future, her and Mark will have sex. Maybe it will be adultery, maybe it will be after an amicable divorce, but I’m pretty certain that now, one way or another, it will happen,
If you hear hoof beats, why look for zebras? 
JavaScript.
This.