Genetic vs. choice?
The results of a recent study (subscription) confirms that maternal but not paternal female relatives of gay men are more fertile than analogous relatives of straight men. The same relatives also produce more gay offspring.
The research doesn't directly answer the question "Is homosexuality determined genetically or chosen?" but argues back to 'gay-by-choice' proponents that a trait tending to reduce reproductive rates of male heirs is apparently more than balanced by an effect increasing fertility of female recipients. The familial relationship through the mother's line suggests this trait might be X chromosome-linked.
Other discussion (registration) of the research cautions that a genetic component of 'gayness' implicated here is insufficient to explain even the majority of homosexuals and may not directly determine fertility. Alternative explanation of a role in enhanced reproductive success is behavioural, in females being more irresistably attracted to men and, so, repeating more frequently the motions that produce children!
(more below)
My own training in biology has always left me with the likelhood of genetics contributing to sexual orientation given the spectrum of realities seen in most sexually related phenomena. From 'competition-cooperation' through hormonal influences to sexually ambiguous genitalia, there's always been plenty of reason to believe genetics is key to sexuality. But I also think that personal decision-making plays into self-perception and how one acts accordingly. In part, these things are matters of interacting degree.
In my mind, 'nature vs. nurture', is itself, more a reflection of human tendency to polarized thinking than it is the reality we see in nature everyday.