So this is driving me nuts trying to remember.
It's the story of a college boy returning home. At college he's gotten a girlfriend. However at home, he had a MOM. A hot mom. A hot mom he wants to bang. The story is basically that the girlfriend he got at college looks a lot like his mom and now he's feeling guilty. He and his mom cross the line. Then the girlfriend finds out. The story ends with the girlfriend being ok with it all.
It's been ages, and I cannot remember the specifics. Sorry.