I thought they proved that being gay was a lack of serotonin and that it should be treated as an illness like depression?
No, according to when I studied biology, the latest research showed that hormone imbalances during fetal development may be the cause.
Also, the more male children a woman has, the higher the likelihood becomes of later male offspring being gay, which is believed to happen due to the mothers body trying to feminise the embryo somehow.
All embryos, whether XX or XY begin development as female, then in the case of XY, due to the action of the sex determining region of the Y chromosome, one particular androgen inhibits female development, and another stimulates male development (I cant recall the name of one of the androgens, and the other is Dihydrotestosterone, but I cant remember which role it plays).
It is believed that in imbalance of such hormones may lead to a highly variable alteration of either a persons sexuality, or even their mental gender, regardless of whichever phenotype is expressed in their genitalia (Also, natural XX males and XY females plus anything inbetween can naturally occur, but is far far rarer than sexuality and mental gender variations).
In any case, no area or association within medical science any longer considers sexuality to be a disorder. Science describes it as a variation, but the exact causes are not known, and any evidence trying to pinpoint the cause on genes has always turned out to be completely inconclusive.