That's not how evolution works! Evolution works at the level of genes, and not individuals.

Yes, of course, though in many cases genes achieve their own survival by boosting the survival and reproduction rates of their hosts.

It's easy to construct a model where a 10% gay population ends up being overall better for a population. Consider this made-up hypothesis: gay people are better at caregiving than non-gay people, so a population with gay people ends up with healthier adults who are able to have more, and healthier, children.

You're invoking group selection here, which is exactly what The Selfish Gene debunked in great detail; given your comment above, I'm surprised that you would make this argument.

From the point of view of the gene, in a society that contained a 10% gay population who were better at caregiving, a gene that selfishly reduced the probability of its host's homosexuality would thrive, because not only would its carriers benefit from the caregiving boost thanks to the other members of society without that gene, they would not suffer from the reduced reproductive potential. Only in the long term, as the gene spread throughout the population, would the caregiving benefits start to fade, and that's not a present-enough change in fitness to apply any evolutionary pressure against the gene (more precisely, it can't apply evolutionary pressure because it depends on the prevalence of the gene in other members of the population; it's a classic prisoner's dilemma situation, and if you're going to take one lesson from Dawkins, it's that evolution always chooses to defect).

As another example, why does Down's syndrome exist? By your logic, shouldn't evolution have optimized that case away? That it hasn't means that changing how the 21st chromosome works is much harder than the impact of having a 1:733 failure rate.

Down's syndrome would be exceedingly difficult to optimize away, because it falls into the category of commonly-reproduced-mutation; it is not the result of code that specifically causes Down's syndrome, it's the result of our genetic material being evolutionarily close to a state that results in Down's syndrome, so whenever something goes wrong, the maladaptive trait is rediscovered over and over. Same thing with most other chromosomal disorders (most of which end up filtered out very quickly, well before birth).

FWIW, that's another common theory about how homosexuality has survived, that normal people are "one mutation away" from being gay (or rather, of having the mutation that makes them potentially gay). Both of these cases still presume, however, that the negative consequences of the trait, when combined with the probability of the trait manifesting, are negligible enough compared to the genetic changes that would be required to move us more than "one mutation away".

Why do you assume that any genetic component to being gay would be easy to change, without having negative consequences elsewhere in the population?

I quite explicitly assumed exactly the opposite. My whole comment on that matter was predicated on the assumption that it is not easy to change susceptibility to homosexuality, and that social mitigation was a workaround.

The main reason I brought up homosexuality at all was that it is often pointed to as a counterexample to the idea that reproductively negative traits are weeded out of the gene pool; I wanted to make the point that evolution doesn't necessarily need to weed out such traits directly as long as it can find some way to control their side effects.

BTW, 100 years ago, pink was a boy's color, and young boys wore dresses too.

Yup, that doesn't surprise me. I absolutely believe that much, if not most, of what signals male/female in today's society is arbitrary. However, I think that the existence of some set of traits that each sex uses to signal reproductive class is very much innate.