Kind of a trick question, and not well considered if you've actually had something like that happen.
The "sleep with whoever you want" (and I don't care/won't ask aspect) comment isn't really the trick. What would most likely happen is that just the idea of going out would be the problem. ie, partner asks:
"Where are you going?"...."Why do you want to go out, it's late?"....blahblah.
Personally I hate dealing with that. The idea of "forbidden fruit" in the sense of cheating isn't really all that enticing. Sexually, well, I love forbidden frut. I have non-vanilla taste buds, and will leave it at that.
Cheating is WAY too much of a hassle than it would ever be worth. If I ever have the desire to cheat on someone, I take that as a sign that the relationship should be ended. I'd much rather be single. If cheating is someone's idea of forbidden fruit, they need to take the time to watch some good, hot porn or something.