Just off the cuff...

Pregnant women have sex because rewiring women to NOT want sex while pregnant would be evolutionarily pointless, not to mention sex is used to bond male partners.

Likewise, sex after menopause. Women still seek shelter from males post-menopause and also want males around for their children and grandchildren's sake.

Men's instincts do not have the abstract concept of "infertile" if a woman appears otherwise sexually appealing.

I'm skeptical of the idea that 50% of young people identifying as bisexual, but if there is truth to this, I imagine that it is largely females who identify this way. And bonding between females is likely to strengthen the 'herd'.

Men's instincts do not have the abstract concept of sperm banks. Even if they did, men are also selective in where they deposit their sperm, and in any case are largely motivated to reproduce through sexual desire. If sperm banks were the only way we reproduced, then we'd have evolved a desire for that.

The idea that there are any humans on Earth that aren't closely genetically related is absurd...

As for monogamy, it's a rational response on the part of males who want to be sure they are the father of their mate's children and who want a better mating opportunities than they might have if it was winner-take-all.

Obviously, I'm not sure of the veracity of any of these, but I think it's fairly simple to explain most of these from an evolutionary point of view. I think it is a mistake to assume that because the rational part of our brain can grasp more efficient ways to spread our genes (e.g. sperm banks), that we must therefore do that if we are wired to spread our genes. The parts of our brains wired that way aren't quite that literal and probably haven't had time to catch up with those concepts.

But then why is it that its pretty much only bonobo apes and humans where this "rewiring" hasn't taken place?

Some apes kill other apes offspring when they take over a group. Others don't.

Some animals such as gibbons and bald eagles mate for life, but most animals are not monogamous.

If evolution principles were so hardwired you would probably expect them to universally

Your question seems along the lines of "if beaks are so successful, why don't all species have beaks?"

All animals are the way they are because of a combination of random mutation and environmental pressure. If a trait is successful in one species, it doesn't mean it will occur in another species. If it does occur in another species, it won't necessarily be reproductively beneficial depending on environment and other factors specific to the speices, so may not ever become a generalized part of the species.

While I normally detest simplistic evo-bio approaches to complex emergent social behavior (particular as it is frequently used to justify shitty, misogynistic behavior), quite a few of those examples could be chalked up to evolution taking a slightly longer view than "get everyone pregnant all the time".

There is probably a strong reproductive advantage for a population as a result of having reproductively disinterested members, and there are individual benefits to sex and sexual desire outside of it leading to pregnancy.

Evolution is not an intelligent process, it's the result of random mutations. The more complex the adaptation the less likely it is to occur. A mechanism for "have sex whenever possible" is much more likely to occur than "have sex except for this long list of exceptions", or even less likely "evaluate this abstract function about genes". Complexity only evolves when there's a real fitness benefit. Evolving a mechanism to specifically avoid sex after menopause would have very little influence on your reproductive success, so there's no reason for it to be selected. The sperm banks example is interesting because that one actually does have a large reproductive fitness impact, but it's also too new and too rare for any sperm donation drive to have evolved. But maybe if the situation continues for long enough humans will evolve to want to donate sperm.

> Why do 50 % of young people identity as bisexual in some way.

I was not aware of this statistic. I really have no thoughts on it either way except - wow!

http://www.out.com/news-opinion/2016/3/11/less-50-teens-iden...

Because evolution is a local optimizer, not a global optimizer, and you can't account for all the factors an evolutionary process accounts for indirectly.

> Why do pregnant women have sex > Why do women have sex after menopause > Why do men stay with women who are no longer fertile

All of these are answered by the following:

A reason evolution would cause these "intentionally" is to help maintain social bonds outside of pregnancy.

A reason evolution would cause these "unintentionally" is because evolution created reward systems to encourage procreation, but never got around to disabling them when already pregnant/can no longer get pregnant.

> Why do 50 % of young people identity as bisexual in some way.

I don't believe this, but if you have a reputable source I'd love to see it.

Anyway, there are plenty of evolutionary explanations for having a population with homosexual tendencies. It could be good for social cohesiveness, or for maintaining a more effective fighting force, etc. etc. Evolution is also imperfect, so it's entirely plausible that sexual desires just get accidentally subverted sometimes. I can't imagine that it's very hard to accidentally trigger the growth of the "wrong" sexual selection hardware during development.

> Why do sperm banks have to advertise for doners

Remember that humans only have a weak desire to reproduce in the abstract sense. It was much easier for evolution to make us want sex (regardless of its outcome) and to raise children.

> Why do people adopt kids, its not their genes

Any time people have an energy-intensive social relationship that doesn't involve their immediate relatives, you have to consider the fact that in the sort of environments anatomically modern humans evolved in, if someone was in your community, they probably had very similar DNA. Therefore, it makes a lot of sense to have the instinct to protect members of the community (especially children) even if they are not directly related to you. In fact, evolution went a bit overboard with this; not only do we want to protect young human children, but also anything with child-like features such as a large head or short legs. Honestly, I think puppies and kittens and the like are a lot cuter than human babies, which suggests that the human baby detector mechanism in the brain isn't particularly well-adjusted and is set up to have an extremely low false negative rate at the expense of a very high false positive rate.

So you're right, "everything I do is to carry my genes forward view" is too simplistic; in fact, you also have to take into account the fact that there is (or at least was) some correlation between social closeness and genetic similarity, so you have a huge incentive to protect your children, a big incentive to protect your siblings and their children, a moderate incentive to protect friends and their children, etc. etc. I'm told this principle has a name in evolutionary biology, but I can't remember what it is.

> Because evolution is a local optimizer, not a global optimizer, and you can't account for all the factors an evolutionary process accounts for indirectly.

This is pretty much my point. People assume that there are some universal evolutionary principles that underlies everyones decisions across species. And that these simplistic principles can explain everyones behaviour.

> I don't believe this, but if you have a reputable source I'd love to see it.

Here is one from the UK https://yougov.co.uk/news/2015/08/16/half-young-not-heterose...

Someone linked to another from the US somewhere in the thread.

I think it is striking how much this a generational thing. And IMHO a behaviour that is probably not hardwired in genetics.