I have met a few people through work and life that have Klinefelters syndrome (xxy) and Turner's syndrome (Xo). While people with these syndromes appear male and female, they do have sexual appearance differences and are infertile. Obviously with the genotypes that they have, they are not your usual male (xy) or female (xx). These syndromes alone are not at all uncommon, and there are many others that are more rare and much more ambiguous.

If it is possible for the genes to be physical causes for sexual ambiguity, then why is it so difficult to think that there may in some cases be good, biological reasons why some people are gay? Many of the folks that I know who are gay have been gay since childhood and have siblings that are straight.