It is nothing short of absurd to credit a psychological phenomenon to purely one source. Since conception, a lot of things have influenced our twisted little minds, be it chemicals and neuron synapses created because of scarring emotional impact or in response to genetic coding - they have the same effect. Some people turn gay, certainly this weighs more on the nurture side - WolfDaddy's earliest memory points towards genetic influence. In nature, especially in psychology, few things are black and white; again this is reflected in the wide range of sexual emotions. From those men who are honestly offended by a picture of a naked male to those who don't even glance at women, there is a lot of room for variation. In a way, everyone is a unique blend of these variables, creating an equally unique result.

P.S. I'm not gay, but I'm equally unsure it's genetic.