From a Washington Post article:
--Gay men tended to have brains that were more like those of straight women than of straight men -- the right and left sides were about the same size, the researchers found. Gay women's brains tended to be more like those of straight men than of straight women -- the right side tended to be slightly larger than the left.
Next, the researchers used positron emission tomography (PET) scans to examine how a part of the brain involved in processing emotions -- the amygdala -- was connected to other brain regions. Again they found that gay men tended to be more like straight women, with a stronger link between the amygdala and regions involved in emotions. Gay women tended to be more like straight men, with stronger connections to motor functions.--
There's lots of information out there about this. Additionally, there are many theories regarding the evolutionary basis for homosexuality. Some say it relieves pressure on the gene pool; more mates to go around. This theory is supported by the presence of various orientations in hundreds of animal species. Others believe that homosexuals fit a niche in that they provide extra support to the family unit. The genes may have been perpetuated due to the historic improvement of reproductive capacities of the mothers, sisters and paternal aunts of these homosexuals, called "kin selection". There are more still. Many of these biological bases may play a role in asexuality as well as bisexuality, transsexuality, and any number of "alternate" sexual orientations or conditions including those still labelled as paraphilias.