“The results, published online this month in the Archives of Sexual Behavior, confirm earlier findings that identical twins are more concordant for same-sex behaviors than fraternal twins are but only modestly so: In men, genetic effects appeared to explain 34% to 39% of the differences between the two twin groups, whereas in women, genetics accounted for only about 18% to 19% of the difference—a finding consistent with other research showing that sexual orientation in women is not as rigidly determined as it is in men.”
http://news.sciencemag.org/sciencenow/2008/06/30-01.html
So, the other 61-66% of gay twins seem to have picked up homosexuality by some other method, like RECRUITMENT.
I don’t doubt that some people are born gay, and that some are also recruited. Gay mens’ predilection for youth is well known since the Greeks at least.
I bet further research would show that it isn’t passed from mother to son but from uncle to nephew (or aunt to niece)—with mom (dad) unwittingly allowing inappropriate opportunities. Didn’t The Who explain this in Tommy?