A Natural Tendency toward Bisexuality

As we gain more & more control to shape our world and ourselves, biological pressures, such as the drive to reproduce will most likely become less central in defining our interpersonal relationships. Nowadays, and certainly in the future, reproduction will be less dependent on straight sex. The focus of sexual relationships of any kind has already moved beyond the purely reproductive, and solidly toward the emotional. This seems to be a direct consequence of the prosperity of civilization. As we free up time and resources that would otherwise have been rationed for necessities, such as food, shelter and reproduction, we can engage ourselves in what would otherwise be considered peripheral interests and self-edification: philosophy, art, personal expression, and non-reproductive sex. Our situations shape our drives. Without the pressure to reproduce or die, more and more aspects of sex move into the optional category. 

If you search "non-reproductive sex” on Google Scholar, you’ll see that there’s been a lot of research on how the environment, prosperous or impoverished, affects our bodies’ reproductive/sexual and chemical make-up.

Poorer communities, such as some in Appalachia tend to have more children, so as to increase the chances of survival in an environment of limited resources. More affluent societies, such as Japan, experience the opposite. Time and energy are freed up for non-reproductive/non-survival pursuits. As a consequence, physiology and psychology both adapt to accommodate the environment.

We have a sexual drive. We have a social-interactive-interpersonal relationship drive. The benefits of a gender exclusive sexual drive lessen when a society becomes prosperous, and able to maintain its numbers with less effort. The benefits of a widened social-interactive-inerpersonal relationship drive remain, and even sharpen with a possible increase in population, due to a decrease in mortality. The inclusion of the sexual aspect is reasonable for the interpersonal drive, in such societies.

This is why I’m of the opinion that both exclusively straight, and exclusively gay sexual orientations will diminish with prosperity, and most people will eventually be bisexual.
This is, of course, dependent on an overall maintenance or upswing in prosperity.
I also don’t think this will happen within individuals, but more so on a generational level. Although simply anecdotal, I’ve witnessed that the current younger generation is already much more physically/sensually interactive without consideration to gender than the one before mine. It seems that in a couple more generations, sexual orientation will become laxer still.