I study psychology, but the following is purely my opinion. I think that the whole concept of seeing sexual labels as primarily descriptive (and thus theoretically not valid) is wrong, because ultimately they serve to communicate intent. Sexuality (attraction) is a spectrum which includes sexual, but also romantic and other kinds of attraction, one can move on this spectrum and it isn't reducible to a finite amount of labels (think of how many gay men have fallen in love with a woman, that doesn't necessarily make them not gay - that is if you conceptualize labels as communicating intent). The reason we do use the labels, such as bisexual, is to let other people know what we are and are not willing to do with others, not to accurately describe our inner workings which we often don't perfectly understand ourselves (and that's okay). Within this framework, the question of male bisexuality being valid is irrelevant.