Flip your argument around. Say that a self-professed gay guy, comfortably admitting to himself and others that he's generally attracted to men and not to women, at one point in his life finds himself attracted to a woman and has sex with her.
Does this suddenly make him straight, in spite of everything else in his life?
You can no more say that an otherwise-straight man, who has gay sex once, is now "gay" than you can say that an otherwise-gay man, who has straight sex once, is now "straight".
There's more to sexuality than just a black and white, this-or-that, picture with nice friendly labels on it. If you insist on a label for your example, then probably most of us fall under the category of "technically bisexual but 99.999% predominantly gay/straight".
You're right that a straight guy who has gay sex once is no longer 100% straight, but there's a difference between 99.999% straight and 100% gay.
And I figure the 99.999% people can just round to 100% when it comes to labelling themselves, for all the difference that one encounter would really make. Ultimately, your self-label for sexuality (if you even give yourself one) is all that matters. Those whom you are predominantly attracted to (physically and emotionally) determine your sexuality, and there's no rule saying there can't be exceptions.
It's a lot like taking this example to a stronger extreme: you're usually attracted to brunettes, but one time you saw this knockout blonde and were attracted to her. This does not in any way change the fact that you're still usually attracted to brunettes.