If I'm friends with the girl's BF then I wont even consider it, no matter how hard the girl persues me.
If some girl has a douchebag BF, or a BF that looks like a chump and know I could probably beat his ass anyway.. then I'll go for it.
I kind of had a predicament the other day with a guy who fvcking threatened me over myspace (can you believe that?) for hanging out with "his girl", even though they're not even dating. I gave the guy my phone number and told him if he was serious we could set up a time and place and handle it.. but he "read" the message and never called. So whatever. I'm not one to step up anyone's toes though, so I just nexted her. There's plenty of girls out there.. but if the girl is a keeper, don't ever let ANYONE get in the way of what you want. Ever. That's my philosophy.