Let me first say that I understand Mangetout’s question to be about sexual preferences, not behavior. (If I’m wrong on this, please correct me.)
As such, it does involve a binary question that can be answered one way or the other at any given point in time and is most decidedly not amenable to the popular but misguided notion of a “continuum”. In other words, at any given moment, one’s sexual preference is either towards people of the opposite gender or towards the same gender; it cannot be both. In this way of looking at the question, bisexuality simply isn’t an option.
So it is in that context that I think we should look at the O.P. To paraphrase, I understand Mangetout’s question to ask: “Can a person’s primary sexual gender preference change over time?”
To that question, I will opine that, based my reading, the answer is gender dependant. There are a great many clinical examples in the literature of women whose primary sexual preference changes in the course of their lives. In fact, it is not uncommon in such cases to see “round trips” as it were, with some women shifting from essentially heterosexual to essentially homosexual and back again over the course of years.
But to the best of my knowledge from my reading, there are no generally accepted clinical cases of males undergoing such fundamental changes. Oh, you’ll see allegations of such, but my clear impression is that they are quite controversial.
Unfortunately, as to the reason why this is the case – as asked in the O.P. – I can only repeat speculations of those who are actually highly knowledgeable in the psychological and biological issues involved (I cannot count myself among them).
One interesting hypothesis comes from Daryl Bem of Cornell. He has proposed the “Exotic-Becomes-Erotic” theory of sexual orientation, which posits that what is seen by children as “exotic” (i.e., atypical of themselves) in the behavior or attitudes of one of the two genders produces a kind of physiological excitement or arousal, which evolves into an “erotic” sexual arousal and thus preference in adulthood for members of that gender.
It’s a fun idea to play with, but it seems rather post hoc to me. Also, it fails to take into proper account the undeniably established fact that genetics plays some role – certainly at least in males – in establishing sexual preference, which Bem explicitly rejects (yet another example of the common (and in my view irrational) bias against sociobiological ideas in the social sciences).
Nevertheless, Bem’s hypothesis does seem to provide a potential explanation for the variability over time of sexual preference in females which also serves as an plausible explanation for the apparent fixity of males’ primary sexual preferences. This idea centers on the fact that there is a considerable social “cost” (or strong social bias against) men expressing or even feeling a sexual attraction to other men, while such “costs” are far smaller for women. The primary reason for this, in my opinion, is that ours is a traditionally male-dominated society where it is considered “natural” for women to be the object of sexual fantasies and interests. That’s why you see so many heterosexual men who become aroused (or report arousal) by the prospect of lesbian sexual activity, while male homosexuality remains the object of often fierce derision.
Given this social disparity, Bem argues, some girls (female children) are more inclined to engage in tomboyish behavior and activities and thus (I gather) are less inclined to see boys as being “exotic”. While from what I have read of Bem he is quite vague about why, exactly, female sexual preference is variable while male sexual preference is not (or at least MUCH less so) according to his hypothesis, my guess is that perhaps he believes that even adult women can continue to pass through the “Exotic-Becomes-Erotic” progression, while that avenue is closed off for adult males.
Anyway, I don’t find Bem’s ideas to be very persuasive, but I thought it might be valuable to at least try to offer one potential “answer” to the O.P.'s closing question.