Why isn’t bisexuality the default?

Probably exactly that. From an evolutionary point of view, same-sex sexual activity is not harmful in itself, but it’s wasted effort. Some species like bonobos are universally bisexual, but that’s because sex serves an additional social function as well as a reproductive one for them.

You just answered your own question. In nature, creatures evolve in ways that benefit their species. That’s the key to survival. Anything else is wasteful at best and fatal at worst.

Ironically, now that earth is suffering from human overpopulation, it is becoming beneficial for humans NOT to reproduce, at least not at the rate it has been reproducing. Perhaps gay sex is actually becoming beneficial in an evolutionary sense? If so, will the percentage of same sex couples increase over time?

It’s more correct to say that they evolve in ways that benefit their genes. It’s very unlikely the percentage of same sex couples would increase for evolutionary reasons, because although that might benefit the species as a whole, the genes that caused it would not be reproduced as much and would thus die out.

It does for humans, also.

For many (most?) people, sexuality is about more than just what feels good physically.

As a heterosexual man, I am “outright repulsed” by the idea of me having sex with another male. But there are also many females I am repulsed by the idea of having sex with—including, for example, females whom I find physically very unattractive, females whose personality grates on me, relatives, and females of other species—even though in each case it might feel similar to sex with someone who wasn’t in one of these categories.

Now, this doesn’t really answer the question of why all the people I would like to have sex with are female—why couldn’t I be attracted to some males, too? But I think it’s not entirely coincidental that the people/creatures I am turned off by tend to be those that I can’t, or at least wouldn’t want to, make a baby with.

I think it’s likely a part of this is cultural (obviously a huge part, at the very least, is biological). I identify as hetero, but if I was raised in an environment where homosexuality and bisexuality were considered completely normal and acceptable, maybe I would now identify as bi. 20 years ago, as a young man, I would have sworn that two men kissing (or more) is totally disgusting (even though at the time I considered myself supportive of gay rights). Now it doesn’t bother me at all, even watching/imagining a more graphic love scene between two men. I’m not attracted to it, but it’s no more objectionable to me than a love scene between a couple I don’t find attractive.

I think this is a key point. Sexual attraction is incredibly complex and while where you fall on the spectrum seems pretty innate, how far and in what ways you stray from that location seems very culturally dependent.

Thus we get very different standards of beauty in different cultures across time and space; we get very different attitudes towards same sex relations, like the way the Romans and Greeks thought about it; etc.

I think this is one of the differences between men and women. I don’t remember ever finding the idea of two women having sex disgusting, and AFAIK that is uncommon.

And THAT part of it is clearly cultural. Cite: “straight”* Greek and Roman men.

*however much use a term like “straight” is when discussing a culture so different from ours.

But that could also be culture - depictions of gay women have been very, very different than depictions of gay men during the 20th century. Certainly in my youth, two men being intimate was generally portrayed as revolting, while two women being intimate was often portrayed as titillating.

It’s chicken and egg, that’s obviously based on what appeals (and doesn’t) to men.

In addition to culture influencing people’s personal understanding or expression of their sexuality, it’s potentially also going to cause evolutionary feedback.

To the extent that non-hetero sexuality is genetic, those who have those traits in a society that disapproves of it are going to be more likely to go into the priesthood, or be the unmarried aunt/uncle, or be the victim of violence, or many other ways in which they’re less likely to pass on their genetics.

To men in 21st/20th Century Western cultures. Again, read some Greek and Roman history.

I thought you were going to say the opposite. In the past and in many societies today gay people are more or less forced to marry someone of the opposite sex, so they are just as likely to pass on their genes.

Yeah, that’s a good point, but of course the ones who who become priests or are stoned for their sins (or whatever) aren’t, so there is some evolutionary pressure over time.

I think you could make an argument that cultural acceptance of homosexuality might result in even more genetic pressure against it than cultural non-acceptance, but I’m not sure if it’s true.

It’s true that homosexual people are less likely to be in heterosexual partnerships that produce children, but lots of homosexual people in homosexual relationships still sire/bear children with their genes, and there are also fewer who join a celibate priest class or decide that there’s just no way for them to be happy in a relationship that could involve children. I’m not sure which effect will dominate, but it seems like there’s probably an interesting paper or two out there on this.

Celibate priests or similar are hardly universal though. Most religions allow and even encourage religious leaders to get married. Modern western society could easily end up being intermediate between the various types of traditional ones on how likely gays and lesbians are to have kids.

There really hasn’t been nearly enough time for evolutionary pressures to apply here. The concept of “homosexuality” as a label to describe a range of human sexuality is a very recent invention, and culturally tied very much to Western European and derived cultures. Other cultures had different concepts about how to categorize human sexuality, and prejudices against minority sexualities would follow those models. Many cultures divided sexuality along “top/bottom” lines instead of “straight/gay” lines, with homosexual activity being acceptable so long as you were the penetrative partner, and not the penetrated. Often, the main concern was that you continued the family line. So long as you had a couple kids to carry on the name, nobody really cared if you spent your evening cruising bars by the docks.

The really strong prejudice against any sort of homosexual activity was mostly a product of late nineteenth and twentieth century social change in the West, and is already falling away. That’s not even a blip on an evolutionary timescale.

In addition to this and multiple other factors: there are also those who contribute significantly to the survival/well-being of their neices and nephews, thereby increasing the reproductive rates of their close relatives.

Your post is generally true, but it doesn’t take a really strong prejudice against something to have an effect. Even a slight cultural preference for heteronormativity (something that I think is a lot more universal than implied by your post) is going to have plenty of effect, and I think it’s had plenty of time to do so.

That’s why I said it’s chicken and egg.

I checked to see if I was remembering right, and what was socially acceptable in ancient Greek times was relationships between an adult man and teenage boy. Unfortunately we don’t have any ancient Greeks around to give their opinions, and you can hardly ask men in today’s society how they’d feel about that sort of relationship.