Homosexuality is mildly heritable. You can't posit that it's hard for evolution to find a way around it, because it has ways. Perhaps not perfect ways, but the heritability should quickly fall it unmeasurably small values. That really requires explanation.
You seem to be making an extremely strong claim: that every heritable deleterious condition must be due to a hidden benefit or recent change. (And in a long-term stable evolutionary environment, would always be due to a hidden benefit.)
What is the basis for this? Why can't we simply posit that some tasks are complex (embryological development), and some tasks are adversarial (countering parasites and diseases), and this explains a large part of heritable fitness-reducing traits?
When you compare it to birth defects, you're comparing one problem against a host of problems. It's worth about the same to evolution to fix the one as to fix all the rest.
This is a good argument.
Do we know or expect that all or most cases of homosexuality have the same underlying cause, or their heritable component does, so that a single localized mutation could eliminate them? Or could there be many different causes?
Also, this feels in need of more quantitative argument. Anencephaly, or even all causes of stillbirths taken together, obviously cause lower fitness for the children than even obligate homosexuality. On the other hand, they don't make the parents spend resources raising low-fitness children whose homosexuality only becomes apparent at puberty. I don't really know what size difference to expect here and whether to be surprised by two orders of magnitude.
You seem to be making an extremely strong claim: that every heritable deleterious condition must be due to a hidden benefit or recent change. (And in a long-term stable evolutionary environment, would always be due to a hidden benefit.)
I don't think he's making a claim that strong. It depends on strength of the heritability and the size of the trait on selection pressure.
It also depends on the extend to which you easily get mutations in genes that produce the trait. Random gene mutations lead to birth defects. It's not clear why you should get homosexuality in the same way through random mutations.
Epistemic status: speculating about things I'm not familiar with; hoping to be educated in the comments. This post is a question, not an answer.
ETA: this comment thread seems to be leading towards the best answer so far.
There's a question I've seen many times, most recently in Scott Alexander's recent links thread. This latest variant goes like this:
Obligate male homosexuality greatly harms reproductive fitness. And so, the argument goes, there must be some other selection pressure, one great enough to overcome the drastic effect of not having any children. The comments on that post list several other proposed answers, all of them suggesting a tradeoff vs. a benefit elsewhere: for instance, that it pays to have some proportion of gay men who invest their resources in their nieces and nephews instead of their own children.
But how do we know if this is a valid question - if the situation really needs to be explained at all?
For obvious political and social reasons, it's hard to be sure how many people are homosexual. Note that we are interested only in obligate homosexuality - bisexuals presumably don't have strongly reduced fitness. The Wikipedia article doesn't really distinguish obligate homosexuality from bi-, pan- and even trans-sexuals. The discussion in the SSC comments used an (unsourced?) range of 1%-3%, which seems at least consistent with other sources, so let's run with that.
The rate of major birth defects in the US, as reported by the CDC, is also about 3%. This counts both developmental and genetic problems, and includes everything from anencephaly (invariably fatal) through Down syndrome (severe but survivable) to cleft palates (minor). But most of these, at least 1.5% of births, were always fatal before modern medicine, and many of the others reduced fitness (via mate selection, if nothing else). Various other defects and diseases, which only manifest later in life, are also thought to be influenced or determined during early development. And so is sexual preference.
(Whether homosexuality is a developmental disorder is not the point; I'm comparing the effect of selection pressure on fatal teratology with its effect on reduced-fitness homosexuality.)
Embryological development is a complex and fragile process, and there are many ways for it to go wrong. We don't wonder how it is possible that selection pressure allows anencephaly to occur in 1 in 4859 births. There are certainly direct causes of anencephaly, explanations of why it happens when it does, but (I think) we don't a priori expect them to be due to tradeoffs yielding benefits elsewhere. It's just as plausible that the tradeoffs involved are against even worse (counterfactual) problems elsewhere - or that there are just no available mutations that don't have these or equally severe problems.
Could it be that linking sexual preference to the biological gender is, for some complex developmental reason, fragile enough that it goes wrong despite all selection pressure to the contrary, that it has no redeeming qualities from the viewpoint of evolution, and that is all there is to it?
When faced with any phenotype with reduced fitness, how can we judge if there is something to be explained - a beneficial tradeoff elsewhere to search for - or merely a hard problem evolution couldn't solve completely? And is there a way to quantify this question, relating it to the known mathematical models of genetics?
Notes:
1. I'm posting this in the spirit of recent suggestions to post more and accept lower quality of (our own) posts to Discussion.
2. I'm going to sleep now and will start replying to comments about 10 hours from now; sorry for the inconvenience.