telekinetic

I think a more accurate topic line would be 'why is cheating WITH someone....'

Since, she's the only one who would be cheating. If your friends with both parties, it's a pretty underhanded scummy thing to do. If you don't know the guy and I'd say it doesn't carry much more emotional impact than hooking up with someone who's single. Unless they're married, then that's a whole other kettle of fish.

Edit: Jinksy posted while I was typing, and I wholeheartedly agree on all counts, with one caveat. I find it ungentlemanly to allow someone to do something drunk that they wouldn't do sober. Once you remove booze from the equation, then the only moral failing (if any) is theirs.