I fucked my girlfriend's mom on the couch while my girlfriend was in the next room passed out drunk. Mamma gave her the liquor!
Cheating. Paying to cheat is probably up there, too.
There's an element of heat in this story.
Consuming Fire of Her Desire - Elements comp entry
When I was about 16, I made out with the girlfriend of one of my best friends. A couple years later, I did it with a couple sisters -- separately, but on the same night in the same house -- and they both were statutorily a few months too young for me. Then a few years after that, I became a boy toy for a much older gay work colleague.