Some very interesting evolution-related research on how traits can be emphasized and preserved is breeding dogs from other canids like wolves and foxes. In the fox study, the researchers only allowed breeding between the animals who were friendly to humans. http://blogs.scientificamerican.com/guest-blog/2010/09/06/mans-new-best-friend-a-forgotten-russian-experiment-in-fox-domestication/
One simple trait, friendliness-- did the fox let you get close and pet it or did it try to bite you and run away? That was it. They continued this over the next 40 generations. And they found that the later foxes looked and behaved more like dogs and less like foxes
They also found that nearly all animals domesticated by humans-- like cows and sheep-- not only behave differently--they also have a different appearance
from their wild cousins. Softer fur, shorter fluffier tails, smaller size, larger eyes, floppier ears, flatter faces, even less strong smelling. We would expect them to act differently because we are choosing based on behavior.
But what would cause them to look less fierce and aggressive, like a more baby-like version
of the wild animal? Apparently, we humans prefer the baby version of the animal more than the adults, and that is what we get. Dogs are like permanently baby wolves and foxes. All changes that make an animal more like what humans want come along with whatever it is that makes the animal more friendly to humans.
What does this have to do with gays? Maybe there is something that produces gayness (which is maybe useful in group survival although not useful individually) in animals and humans that travels along with other specific traits that get passed along. Like the aforementioned eye and hair color. And you can therefore choose to be gay or not, exactly the same way you can decide to be born with blue eyes and red hair....