Gwen Cooper/Owen Harper, Torchwood.
She cheated on her boyfriend, who was really great and didn't deserve that shit, and Owen was an ass. I've never liked the hints about her and Jack either.
Rose/Doctor 10, Doctor Who
I can see why she's attracted, but him? She's an immature teenager.
Kara/Lee, Battlestar Galactia
She was engaged to his brother.
There are more, but that's all I can think of right now.