I get the impression that people define one's sexual orientation based on who they have sex with, rather than by to whom they are attracted. I addressed this once before in my Kinsey post.
Sexual orientation is determined by who you are attracted to. You could remain a virgin your whole life and still be straight, gay, bi, asexual, or pansexual. It's never about who you've actually fucked.
A man who considers himself straight, but who gets drunk one night and lets his male buddy suck him off, can still be 100% straight. Being drunk may have allowed him to get over any sort of uptight notions he may have had about being that intimate with a guy, but if he has no attraction to men, including his friend, even if he's received oral sex from the guy, he's still straight. Furthermore, even if he gave it the old college try and sucked his friend's cock, he could still be straight. Experimentation doesn't indicate an orientation, either.
This is one reason why we need to respect people's opinion of what their own sexual orientation is. For us to tell that man that he's gay or bi simply based on our knowledge of a one- or two-time same-sex experience, is presumptuous and arrogant of us.
There are times when you can feel like the person is kidding themselves when they refer to themselves as straight when all evidence seems to indicate otherwise. Even in those cases, I believe we should leave it up to them to define their orientation. Partly because we don't know the full story, and partly because it can be a rather personal, and emotional moment to tell them that they're gay, or bi. Especially if they seem oblivious somehow.
Yes, it's possible to be oblivious as to what your orientation is, or at least be questioning it. We don't really want to rush someone who is trying to sort things out and figure out a big part of who they are. When I was young and realizing that I was gay, I had to come to terms with it and accept it. Had someone come along and told me I was gay, I might have freaked out, and not just because I would have felt "outed." It might have also bothered me, at the time, that it was that noticeable, or I might have been embarrassed that they figured it out before I did. I'd also kind of like them to mind their own damn business.
So, if I were to fuck a woman, for whatever reason... and I really can't think of any reason why I would want to, but if I did, I'd still be gay. Even if I managed to get an erection; even if I managed to love her in some capacity; and even if I did find her attractive, I'd still be gay. Seeing someone as attractive doesn't necessarily mean that you are attracted to them. I think George Clooney is attractive, but I'm not attracted to him. I have no interest in fucking George Clooney. Sorry, George. I also think Charlize Theron is attractive, and, again, I don't want to fuck her. Sorry, Charlize.
Next time you want to try to define someone else's orientation based on their behavior, remember that they could just as easily do that to you. Just let them be who they are. It doesn't matter if you think they're kidding themselves. If they are actually some other orientation than what they claim they are, they'll likely eventually come around. If not, it doesn't really matter to anyone but them, and who they choose to fuck, does it?