I can understand that for a woman, given the presence of artificial lube, but an erection is not something one can fake ... is it ... ?
I think if beer (which reduces inhibitions) unleashes homosexual behaviour, then logically, in was the inhibitions which kept it from manifesting at other times.
And yes, I think it is much more likely that there is a gender-orientation spectrum, with only a tiny minority of people being 100% gay and a larger but still small minority being 100% straight.
My guess would be that at least 80% of the population, in their natural state, would experience attraction to both genders, often skewed more to one gender than the other.
If this were not the case, we would not need such violent social controls in the culture to suppress same-sex attraction, would we? 
Jenny