Thinking about sex

Regarding Nurture/Nature debate over the causes of human sexual orientation...

If people are programmed genetically to have an orientation, this raises two questions. First, if orientation is set to an initial condition, why is it so changable thereafter? And second, what purpose - if any - might be served by genetic sexuality?

Sexuality is extremely susceptible to change, but the mechanisms and causes of this change are highly mysterious. We know that people develop new tastes and change old ones, but strangely, it seems impossible to deliberately create such a change. Contrast gay men who unexpectedly fall in love with a woman and start a family with the complete failure of the ex-gay movement.

If sexuality is preprogrammed, what is the point of it being randomly reprogrammable later? Conversely, if sexuality is fluid and constantly changing, what purpose is served by having several different initial position?

There may indeed be no panglossian purpose to genetic sexuality. Perhaps there was a purpose once, which is now out of date, but the structure remains. Or it may be some kind of by-product.

One possible solution is that sexuality is indeed preprogrammed, and it does serve a species-level purpose to have multiple sexual orientations in a society, including those which do not result in propagation. However, the 'glue' that binds a person to their programmed sexuality is weaker than would be ideal. This some people become detached, and float into a new orientation.

This reasoning requires that genetic adaptation (however imperfect) takes place in progeny with a view towards survival of the species, as opposed to the progeny of an individual. In other words, a simple 'selfish gene' model cannot be used. It may also require a notion of teleology to be reintroduced to evolution studies, which would be pseudoscientific.

Assuming variation in sexual orientation is not unique to humans, and assuming the purpose (whatever it is) of this variation is the same in humans as in nonhumans, they we have to ask: what is the point of a bisexual fruit fly? Let alone a gay banana?