You say this is a "moral conundrum", but it's really not. The moralistic thing to do is to not do it. Having sex with them would he highly immoral.
With that said, I probably would. If it's just a one night stand, we're already at my place, and we've already started fooling around, I don't think stopping at that point will do much good. Also, it won't be all that bad for me, since I'll never see her again and it was her decision to cheat. In this scenario I'm assuming I don't notice the fact she's married until we're at my place, because if I knew she was married from hello then I wouldn't bother, since I don't actively seek married people.
With that said, I probably would. If it's just a one night stand, we're already at my place, and we've already started fooling around, I don't think stopping at that point will do much good. Also, it won't be all that bad for me, since I'll never see her again and it was her decision to cheat. In this scenario I'm assuming I don't notice the fact she's married until we're at my place, because if I knew she was married from hello then I wouldn't bother, since I don't actively seek married people.