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Douglas_Knight comments on When does heritable low fitness need to be explained? - Less Wrong
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But they don't. Anyhow, that's still a 25% fitness cost, which is huge.
From the perspective of the parents of the gay couple, it's a 0% fitness cost, though.
Irrelevant.
We are discussing the dynamics of genes. If the gene has low fitness, it will be purged from the population and the rate of homosexuality will decline, even if the grandparents have full fitness. By using a donor, we are re-randomizing. Instead of a parent with the gene, we are replacing with a parent with a 50% chance of having the gene.
Similarly, what if two people with achondroplasia have children, repeatedly, until 2 survive infancy. Then the parents have fitness 1, but 1/4 of their children were homozygous and died in infancy. Their children are uniformly drawn from the remaining 3 children, consisting of 2 heterozygous, 1 homozygous normal. So the gene frequency has gone down from 1/2 in the parents to 1/3 in children. The parents have full fitness, but the gene has fitness only 2/3.
I don't actually know if they do or not, but why don't more people do that? Maybe they're afraid the donors will expect or demand some rights or involvement in the child's life, which isn't a problem with anonymous or hired donors.
Agreed regarding the fitness cost. That makes chaosmage's comment correct, although I didn't realize it at first; his comparison with asexuals led to me to think his proposed mechanism was simply homosexuals raising fewer children, whether their own or not.
You might as well ask why people adopt strangers as ask why they half-adopt by using strangers as donors. The answer is that people are adaptation-executors, not deliberate fitness maximizers.
I hear about people asking for donations from friends more than family, which seems to me to be a bigger risk. Being on bad terms with family because of orientation probably is a reason, though.
People have always sometimes adopted the children of near relatives who couldn't care for them, at much higher rates than adopting strangers; that's fitness increasing. It's not a stretch to imagine that crossing over to sperm and egg donations.
Both friends and family have the large advantage (at least I imagine it to be so) that they are known quantities: you know them well and can evaluate them as biological parents just as you would a potential mate. I wonder why sperm banks don't seem to offer the sperm of people with known qualities at higher prices: not just screened for diseases, but known to have strong positive traits like good looks and intelligence.