“His study on sexual orientation, out next month, says that gay and lesbian parents are far more likely to have children who become gay. “I’m trying to prove that it’s not 100 percent genetic,”
Here is the problem in a nutshell, trying to prove that it is not 100 % genetic. There is no evidence whatsoever that it is genetic but somehow they have argued loud enough and long enough that the argument begins from a point that was never substantiated.
Only a fool tries to prove a negative.
We have hundreds of thousands of examples of nominally straight men indulging in homosexual behavior, and for one reason. Like most males, they have a propensity towards indiscriminate sex with anybody willing. (Not saying this is a mature characteristic !)
The chance of it being genetic is vanishingly small. The chance of it being by choice is quite large.