You properly point out that "preference" (or "attraction") is not the same as "behavior." There have been studies, reports, and articles published lately (I tend to read some of the more popular science journals and magazines, rather than the psychology journals) that there have been found distinct differences in brain morphology between practicing homosexuals and heterosexuals. So there may be some truth to observations about homosexuality.
However, the contention that homosexuality is driven exclusively by genetics can be tested. Identical twins, separated by birth, share an identical genetic makeup. If it is found that one separated twin is a practicing homosexual, or maintains homosexual attractions, then it stands to reason that the other twin would also have similar attractions, and should in 100% of the cases. In fact, the findings were nothing like that. I recall the number reports was something less than 20%.
So clearly, there are other factors influencing both preference and behavior. I'm not a psychologist, and I didn't sleep at a Holiday Inn last night either. So I will not pretend that I know a lot about the psychology of human sexual behavior (other than red-heads are hot!). But I do know from my education and professional scientific background, that the "all" and "never" cases rarely occur in nature.