Well I think its mostly a matter of attraction. If you're not into guys...or their cocks, then there'd be no desire to suck dick.
But I guess that'd be obvious.
If you're talking about a threesome scenario or what have you, I think a lot of it may have to do more with the stigma against homosexuality that still exists among a lot of straight men.
There's this strange perception that women can dabble and experiment while remaining straight, yet men are either gay or bi the moment they cross that boundary.
Its strange, but I think that might be what you're talking about.
I for one identify as straight, in that I don't find men attractive at all (I seriously don't know what women see in us)...but I'd be willing to suck some mad cock in the right situation.
Not sure where you're getting your data, but the gals you describe sound suspiciously like porn stars... who are, after all, paid to act like sucking a dong is the best activity they've found since people got kicked out of Eden. Having actually talked to many real live women about their sex lives, I can tell you that there is a range, and most women are not into sucking any old dick that happens across their path. The majority are not all that into fellatio for its own sake, but may be interested in the person the dick is attached to, and if she is interested in him and he is interested in having her suck him off, then she's happy with that. Among those, there are tons who are not interested in swallowing at all, and many who are just tolerant of the act but really enjoy the pleasure they give their partner.
If you were to take a large-sample poll (I'm sure someone has done this) of straight and bi women and gay and bi men, I would be willing to bet that the gays and bis are more into giving a blow job than the straight women simply because you would get a lot more sexually inhibited folks in the straight group. You'll be hard pressed to find a straight dude interested in giving a random blow job, but would have a lot more luck looking for dudes willing to give a BJ to a specific guy or in a specific situation.
Two great and well reasoned replies, thank you.
Yes, its a fact that a large amount of content on here is fantasy and yes, other sites where there are videos to watch are 95% porn stars getting paid to enjoy oral sex.
I see myself as being very open minded and see any sex as being a gift we should enjoy.
I want to suck a cock, but its all experimental, probably a large amount of fantasy but would still like to have the opportunity.
Because not all guys are gay or bisexual. Very simple. No one is going to suck dick or eat pussy or whatever with someone they aren't in to. Even in a threesome scenario, a MFM.... I'd be willing to do that, but I have zero desire to do anything with the other dude. I don't want to suck him off or stroke him or kiss him, nothing. Because, I'm not into guys. That's like asking why lesbians don't like giving blowjobs. They aren't into dudes. Same thing with strictly straight women, I bet most of them love having their pussy eaten, but that doesn't mean they want to eat pussy.
I have a friend that is (well, was) a swinger. She LOVES giving head. She and her husband went to many swingers parties and had lots of sex, but she never fooled around with another woman. In her words, "The only pussy I have any desire to taste is my own".
When the debate is lost, slander becomes the tool of the loser. Socrates 
