> 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.