I'm starting to think LEGO is evil

I think this is actually a pretty reasonable statement, as long as it includes the caveat "on average" or "most of the time". Just as Down's syndrome isn't a huge problem for a population -- as long as it stays relatively uncommon.

A population comprised largely of people with Down's syndrome would likely be poorly adapted, and that's probably the case with a population comprised largely of gay people or transgender people as well. (Obviously, this is complete speculation, so I could be utterly mistaken.)

But yes, a population with a certain percentage of gay people could be better adapted just for having them, or alternatively, it could be better adapted because the same genetic diversity that leads a percentage of the population to be gay could be desirable in other ways.

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.

Is the mutation which causes sickle-cell anaemia a "good mutation" or a "bad mutation"? It increases reproductive fitness in places where malaria is or was common, so it must be good, in an evolutionary sense.

How many deaths has it caused once the population of people carrying the haemoglobin gene mutation migrated to a location without malaria? Is that mutation now "good" or "bad"? How do you incorporate those numbers into your statistics?

Is the loss of eyesight a deleterious mutation? Definitely for a bird of prey, but not so for cave-dwelling creatures living in absolute darkness. For that matter, some people are attracted to people who wear glasses (and wearing zero-prescription glasses is such a turn-off!), so it might increase reproductive fitness.

Evolution doesn't know the future. If a population loses genetic resistance to a disease that's seemingly extinct, is that a "good" or "bad" mutation? How long does it take to judge that? After 1,000 years, should some thawed carcass reintroduce it and the species become extinct, does that count finally as a bad mutation and a single death?

For a real world example, consider the birds of New Zealand. They filled ecological niches which elsewhere were filled by mammals. Were these good mutations or bad ones? And when rats and weasels and cats and more were introduced to New Zealand, helping make many of those species extinct, then did those mutations retrospectively become deleterious?

If a genetic madness affects the leader of the US Strategic Air Command to issue orders which end up nuking a dozen Soviet cities, then what are the other cases which make that average out to one? If the nuking didn't occur, then what would the average have been?

What of a mutation which causes a speciation event? Is that a good mutation or a bad one? It's better for one environment and worse for the other.

There's a 10^-9 chance (1-in-a-million) that a "bad" mutation will mutate again back to the "good" form. With nearly 7 billion people in the world, that almost certainly happens a few thousand times every generation. In a generation we may be able to cure some genetic diseases through genetic engineering, so a "bad" mutation can be fixed.

With all those in mind, I can't figure out a way to get the numbers to come out "1" unless the definition of deleterious is defined to make it come out that way.

Personally, my suspicion is that homosexuality is more directly linked to a positive physical trait in the individual, though I don't have much to really back that up other than a vague sense that kin selection effects in evolution are rarely as strong as direct expressed ones. But yes, the "gay uncle" effect could explain it, too, and it's definitely an interesting enough phenomenon to be worth keeping in mind.

[1] In fact, I probably shouldn't react as negatively as I do against most invocations of the group selection argument, because oftentimes the points would be valid if expressed as kin selection arguments instead.