I would think it would be a malfunction of one of the endocrine glands. A malfunction not a genetic problem. Here's my quandary: I'm thinking that if it is purported to be genetic, why would heterosexual parents have gay children?
If it is, is a malfunctioning endocrine gland a matter of choice?
As far as heritability, not every heritable trait is expressed that is passed down. Traits skip generations.