The sight of a big cock is very important to "most" straight men Evolutionary biology tells us that men are very attracted to the sight of a big penis. That said, it has nothing to do with a man's overal sexual orientation. Men can be very straight and still enjoy transsexual women. It's just a mere fact that hetero men like seeing big dicks and interacting with big tits -- period. Just because many straight men are aroused by the sight of a big penis doesn't mean they want to suck Johnny's dick.
What makes a man more bisexual is his desire to interact sexually with another man's genitals on top of stronger sexual attraction to the male body overall MINUS visual excitement from the sight of a big penis (not every single straight men is aroused by the sight of a big penis, but evolutionary biologists tell that the vast majority of straight men are).
I'm a bit gayer than some dudes on this thread because I have some attraction to male ass and male body. But I don't consider myself bisexual because I am not aroused by sucking dick or having a dude suck my dick (not at all arousing to me). I can get aroused by a few men visually, but I don't want full-on sex with them. I describe it like this: women who like lap dances by other women, but don't really want to be with a woman. It's hard to explain.
Always remember that no two straight men are exactly alike. Every man has a unique sexuality.
I do understand that some straight men who suck transwoman's dick seems gay, but they're only lusting after the cock -- not the overall male body. It's hard to explain.
I will come back to the evolutionary biology points later.