Hasn't happened. At least, I hope it hasn't happened.
I've got a pathetic romantic history for a twentysomething--I had a boyfriend briefly in college ("Bob"--nothing like his real name). One boyfriend, ever. It didn't work out, mostly because we had very different ideas about what a relationship should be, although he's a great guy (smart, hardworking, funny, kind). My brother was the best man at his wedding and said the whole thing made him sad because he couldn't help thinking that I would have been better for Bob long-term than the girl he married.
I'm sure I'm mostly suffering from One Boyfriend Syndrome, and I wouldn't worry about it so much if he hadn't been the only one who ever showed any interest in me (I'm great at being friends with guys but it never occurs to them to ask me out).