Ookay, well, speaking from personal experiance, the reason a lot of guys think girls can't race is simple:
Every car show I go to, there's like, me and two other girls there that are actually there -for the cars-. The rest of the girls there are in little hot pants and tiiiiny tank tops flashing their !@#s around like they're Pammy Anderson's long lost sister.
And those girls make -us- look bad. In fact..
I was at one recently and two of them were trying to mack on my boyfriend. Bleach blonde, fake nails, whole nine. (no offense to bleachies, but these girls were NASTY). They saw his car (an RX7 with a killer kit) and like, locked target..
I stood there and listened to them flirt with half of my attention (the other half was directed on a niiiice Supra that my friend had brought down), and before long, I was about to smack the hell outta them.
So. (sorry for that long winded round about post). In conclusion: Some people think girls can't race because of racersluts.
And that tailgaiting/cell phone/makeup thing doesn't help either. :\