Personally, I don't care if a girl has a boyfriend... as long as I don't know him, I'd fuck her.
Now, I wouldn't make it an ongoing thing though.. either he or I go.
Actually, I have a funny story in this department.. In college I was sleeping with this girl who had a boyfriend back home (she was a freshman, he was a HS senior)... We had been dating for a few months, and she kept saying she was going to get rid of him.. but never did. So I gave her an ultimatum, which of course passed.
This was pre-instant messenger days, so I found him using 'talk' the old unix command, and told him I'd been fucking his girlfriend for the past several months; was breaking up with her; and just thought he should know.
He dumped her the same day, and he and I went out and drank beer.
