I have never been one to sleep with a girl that I know is married or has a serious boyfriend. There are plenty of other girls out there, no need to ruin some guys situation.
ALTHOUGH!!! And this is a big ALTHOUGH
Every hot girl has some guy that she could MAYBE consider a boyfriend. You have to think that all hot girls are having sex with someone... Who knows how serious it is?
Also, most hot girls have guys that do things for them, like take them out to dinner, borrow their cars, follow them around at the club...
These guys could be considered boyfriends. It depends on who she meets. If she meets a really cool guy (YOU!) she won't bring up all those orbiter guys, but if your approach is lame, she'll give you the "boyfriend" response...
And like others have said, it could be a shit test... My normal response:
HB: I have a boyfriend.
ME: Cool, I actually have like 3 girlfriends right now, but if your lucky, I'll make room for one more...