This has happened with me many times. I didn't plan it, and now I've learned to sidestep the situation because I always felt really funny about it after.
I work with men a lot, in a construction setting, and I've seen that lots of husbands/wives are kind of like roomates after a while it seems, and the men get a lot of the headaches of making a living but they don't get blowjobs at home or nsa sex when they want----i don't think it's easy to be married so I'm not judging. I doubt the wives get flowers for now reason and boxes of chocolate and date-nights once a week. I think it's hard for both of them and frivolous sex is maybe not part of the relationship any more.
In my case, listening and being funny and complimentary was the combo-thing they didn't seem to get at home. That must have made them relaxed enough to get to the point where they felt ok about a guy being that close to them. In every case I looked up and saw their eyes were closed, probably fantasizing about a woman going down on them.
It was always a very sexy and memorable experience, but I don't let things go that way anymore if I see the potential because I guess I'm older and hopefully more conscious of other people's property and vows. Plus, I didn't feel great after it, just slutty.
But a straight unattached guy is a different story, I've not given up on that. Thats fun.
I think the constant "no straight guy would let another guy blah blah blah" is wrong, because of one thing: the internet has taken a lot of previously 'taboo' things and presented it in ways that take the fear out of it for a lot of people. Many people are not complex thinkers and they want to get off. Other people might have a problem with it. People are different.