While some people are saying "well if the marriage is screwed anyways..." (which you're helping to ruin anyways, hypocrites) I don't think that's the only point. Its one less possible std, its one less possible explanation to a pissed off spouse, and its one less possible story to try and explain to people you know. I feel like one night of potential sex (which if we're being this hypothetical, isn't that hard to come by in a bar full of drunken idiots) isn't worth much, but hey, some people like to fuck anything that asks them, so reason isn't going to persuade them.
If you're going to further the pain of another man, then at least have the decency to tell him, otherwise you're nothing more than a spineless troll who deserves nothing less than cancer. (You're still a spineless troll though, just maybe not worthy of cancer)
I know some people think sex is necessary (it isnt) and its great (good for you scooby doo), but I, like some people, have standards. I actually have the self control to say NO, and I think that's important. I've seen some people that completely break under sexual pressure and can't say no, and I believe that is a sign of a pretty weak person to be honest.