It is now well established that same sex attraction is not volitional. In The Microcosm Within, the incompatibility of any adequate theory for the persistence of homosexuality within the standard Darwinian construct of natural selection is fully discussed. The evidence points to same sex attraction as a genetic event representing the conservation of an original cellular capacity for self-reproduction brought forward over time as a conserved core process and expressed throughout the animal kingdom as same sex attraction.
More recently, there has been consideration that same sex attraction might be under epigenetic influences. A relationship between epigenetics and homosexuality have been cued from a 2012 study that proposed an experimental model in which epigenetic markers relate to same sex orientation. This theory was offered as an explanation for some interesting well known observations; the tendency for homosexuality to run in families, the fact that only 20% of identical twins express same sex attraction, and the fact that no ‘homosexuality’ gene has ever been identified.
The proposal of the research scientists in that study is that epigenetic markers are shaped by androgen signaling and become a factor in gonad development thereby molding sexual orientation. In effect, the underlying genes are modified by epigenetic factors based on the differential experience of the fetus to the stress of maternal hormonal levels. These markers can then be transferred to successive generations accounting for the familial tendency. It is not merely the absolute level of the androgen signals that might matter, but the underlying sensitivity of the developing cells to these signals. This might differ from individual to individual based on genetic architecture or there may be an inherent difference between males and females. The effects of these transmitted epi-marks need not be specific to the gonads, instead affecting areas of the brain, so that there is no difference in the gonads themselves.
Not everyone feels that this type of theorizing is particularly important since it is clear that same sex attraction is simply part of the human condition and our inherent variability. Yet, this sort of investigation and speculation does serve to emphasize that same sex attraction, what ever its causes, is not subject to the simple rules of natural selection that underpins Darwinism. Further too, if such a trait with its consequential impact on reproduction escapes natural selection, then a theory of evolution as impelled by natural selection needs to be reexamined.