I have a hard time believing in strict heterosexuality. Or strict homosexuality for that matter. I am fully convinced everyone's just different levels of bi.
I've had sexual contact with so many of my 'straight' friends, or had 'straight' friends try to initiate some form of sexual contact with me, that either something about me specifically attracts closet cases or people are generally much more flexible sexually than we tend to publicly acknowledge. I think it's the latter because I've also had multiple friends and boyfriends who identified as gay admit that they've had sex with women too, even after they came out.
I mean I'm sure there are "true" fags and "true" breeders but if anything I think they're in the minority. They're not as common as we think they are.