The whole nature/nurture business is a complicated one, and when you start to talk about sexuality only gets more so. As a physiology professor said to me once: “You’d like to think your sexual choices were made at a level higher than the hypothalamus.”

But it seems little doubt to me that there is a biological component to sexuality, as appears to be confirmed by a study showing that the more older brothers a man has, the more likely he is to be gay.

The mechanism by which having older biological brothers affects male sexuality remains unknown, but the most popular theory is that it reflects the way a mother’s immune system reacts to carrying lots of male foetuses.
As males have a Y chromosome and females do not, a mother’s body may be more likely to recognise a male foetus than a female one as foreign and generate a strong immune response.
Other research has shown that this response can strengthen with each subsequent male pregnancy. This may affect the way that the brain develops sexually.

I do like the challenge this presents to the religious sorts: if God made everything, doesn’t this mean he made homosexuality?

But it also makes me think about the problem of trying to adjust your sexuality for your politics. I knew a few woman in my university days who decided “to become lesbians” for feminist political reasons. I respect the argument at an abstract level, but I also saw some hideously exploitative relationships result from it – almost as bad as those resulting from women consciously “just experimenting”.

I’m heterosexual. I don’t know why that is, but that is just the way my sexuality goes, quite strongly. But that makes me realise how horrible are attempts to force those otherwise inclined to conform to some form of social norm.