That makes no damn sense.
Why it doesn't make sense, it seem pretty easy to explain from my point of view:
vs
You clearly not turned off.
Sure, I'll easily admit that I am turned on at idea of doing the stuff I know I love doing with a woman with a man because again in my head, doing it with a woman = I have to pay for it which is a bit of a turn off for me whereas doing it with a man = it's free which is a pretty big incentive. But when I'm actually doing it with a man, it's just not enjoyable.
You do not have the same sexual desires for all women.
Sure, that's 100% true. But on the other hand, I never see a guy (no matter how he looks) and think: I want to do stuff with him. And actually seeing the picture of a guy before meeting him is a huge turn off for me. But guess I'm not straight.
There are bisexual men that only enjoy giving or receiving oral from one gender, because it doesn't turn them on. That doesn't make them less bi.
Sure that's true. Now imagine a world where a guy only enjoy receiving oral sex from men or women but the only way he can receive oral sex that way is by paying for it. On the other hand, he can easily find men for free that will want him to give them oral sex. Do you think that guy will never entertain the idea of meeting men to give them oral despite knowing that he won't enjoy it? Possible, but I'm sure there are at least a few guys in his situation that would. I'm one of those guys in this situation.
Another example: men or women in prison for life that are straight. Do you think they never entertain the idea of doing sexual stuff with people of their gender at some point?