One CAN be 100% one way or the other, but with some individuals that may fluctuate over time to allow for some variation I think, I am 48 and have never had anymore interest in putting my dick in a female than I have of putting it in a horse. If a person says 99%:1% either way, they are just saying it is very unlikely they will ever have the other kind of sex, but they do not rule it totally out as I do.
I like women, I can love them as both sisters and friends, but I just am not turned on by them, same as with horses. I have been with a few females, mostly because I was young and EXTREMELY horny and had no other outlet except to masturbate, and hell, they practically raped me.
But, I would never have chosen it, I did not enjoy it, did not want to touch them, other than to get the rocks off and leave. If alcohol had not been involved even those few times would never have happened. I suppose had I answered the poll back then I would have said 90% gay and 10% straight. But, in my deepest being I am a gay man and happy about that. I did not choose it, but I would choose it if I could.
So, maybe that answers also the question of how a 100% straight man can fuck a guy and yet maintain he is 100%. It sounds illogical, yet I still believe I am 100% gay. As far as emotional ability to be attracted to a person I am only attracted to guys, so that while I might have had sex a few times with females in the distant past there was NO chance of anything further happening, not even a second encounter.
I think the 100%'ers are part of the reason there is so much friction about sex, we are seen as rejecting the other side without any consideration of their views. But, I do not dislike anybody based on their sexuality, if they are cool with me I am cool with them. It is the threatening macho hets that think it is OK to publicly use terms like faggot as derogatory that piss me off. Or, the women that are so hateful because they are rejected by men that prefer other men.
There are places where life is mostly gay, but straight men and women are welcomed into every gay bar I have ever been into, you cannot say the same for the other side. Unfortunately I live in a region where the nearest gay venue is over 250 miles.