I was just in this situation.
There was a temp where i work, (I dont usually mix pickup with work but she was only there for a few more weeks). I talked to her, ran my routines, did a few things that showed her I wasn't like everyone else, that I was interesting, yada yada... She had a dream about me later that week and basically told one of my freinds she wanted to make out with me.
well anyways, I successfully F close this girl, and we see each other a few more nights after that. This was only supposed to be a hook up. Well I realized I started to care about this girl, we really had quite a few things in common, and the sex was great. The issue was, she had a boyfriend. dun dun dun! I never really let it bother me too much. so fast forward 4 weeks, we decide to start dating instead of just meeting up for sex, I dont deny it because I've started to like this girl. fast forward again 6 more weeks (this would put us at 2 weeks ago) She wants to hang out, I tell her I might be able to, but probably not because of family plans. I meet up with my family and do our thing then go over to my friends house for a few hours since i haven't hung out with them in a while. She creates drama out of it and it becomes a huge problem because she says she loves me and wants to be with me, but she loves her boyfriend too and can't leave him. so i decide to stop seeing this girl because quite honestly I don't want a serious relationship.
It took me roughly 1 week to realize this, and only when I went out and started picking up again did I realize this was the famous oneitis. I loved the sex, not the girl.
from what I've seen, girls with boyfreinds can make great fbuddies, but I wouldnt take it any further or longer than that again.
.02