I think people are conditioned into their sexuality based on external (and in some RARE cases) internal stimuli.
Biologically the 'Default' orientation is 'straight' (hence the term "straight") but depending on how one was raised, who their parents are, and what they're exposed to, that can change.
Being gay isn't natural, but it also isn't a 'terrible' thing... More or less it's an expression of your past experience, and what you have come to find appealing in a mate.
Girls want to marry their fathers, and boys want to marry their mothers.
It can also be like "Goth" or "Nerd" or "Jock", what have you. Goths tend to gravitate toward other Goths, etc. once together, they sort of feed of each other, and become more exaggerated versions of themselves.
I believe homosexuality is no different.
(I don't mean to equate homosexuality to a fad, but merely conditional behavior)