Consider musculature. One gene may regulate the total mass of muscle, one or more others may regulate the ratio of fast, slow, and medium twitch fibers. Or there might be three genes, one responsible for a level of production of fast-twitch muscle fiber as a response to exercise, another for medium-twitch fiber and another for slow-twitch fiber.
Now, consider how complicated we all know sexual behavior to be. How many of your values and emotions are involved if you consider all the different ideas related to sex? Is it likely that one gene alone determines your sexual preference? Cochran assumes so:
"One expects natural systems to work pretty well, because of these endless generations of pruning and refinement. And again, working means replicating - and, of course, the functions that support replication. Good, not perfect: mutations happen, almost all bad. But the mutation rate of any individual gene is always low, and natural selection keeps weeding them out: mutation-driven diseases are never common. Certainly the most common ones known are several hundred times rarer than homosexuality (which, by the way, doesn't even look particularly genetic in origin - MZ twins are 80% discordant). In fact the evolutionary cost of homosexuality is about as big as the sum total of _all_ known mutation-driven genetic disease. It can't be cause by mutation - too common. Let me make this very clear: if one in ten thousand men were homosexual, homosexuality would not be an evolutionary anomaly. A few genetic diseases are that common. But when, say, 3 in 100 men are homosexual, it's a huge anomaly."
First, it seems more likely that homosexuality works according to a switch, as (I think) Jessie has already suggested, wherein behavior that is advantageous to reproduction is switched to homosexuality. Maybe the gene(s) responsible for cooperative behavior in asexual same-gender relations also has (have) a role in sexual behavior?
Second, the evolutionary cost of homosexuality is a term I did not find defined in that article, but I did notice that on the second page of that collection, Cochran mentioned that "homosexual men have low fitness, around 0.20 in recent US surveys." If my memory of conventions in statistics is correct, that 0.20 represents 20% of homosexuals passing on their genes to the next generation? That does not suggest to me an expectation that homosexuality vanish from the population. As long as the total population grows at a particular rate, I wouldn't expect changes in its composition.