It's bad to have sex with your friends' significant others.
Tom is hanging out with his friend Bill and his wife Susie and realizes he enjoys her company.
Tom wants to be around Jill more.
Tom tells Bill and Susie he enjoys their company as good friends.
Tom asks Susie to sleep with him later that night when Bill isn't around.
Bill and Susie are flattered and hang out with Tom more often.
Bill finds out and is angry at Susie and ends his friendship with Tom.