It's not bi at all because the sex organ doesn't not define the person as a whole.
A girl having a penis doesn't not make her male, just like how a guy with a vagina does not make him a girl (also, ick)
What is the attraction of dickgirls? Simple: dicks are awesome. I have one and I like it very much. Dickgirls take that organ, and transplant the things it does and likewise the feelings known about them and attaches them to the sex that I'm attracted to. Win-win.
If you've jerked yourself off (and we know we all have, often) what's the difference between that and giving a handjob to an attractive girl? If anything, that's even more of a turn-on.