I would guess some combination of recruitment, grooming, alcohol and drugs, parties, government schools and other cultural brainwashing, peer and adult pressure (or rather suction), monkey see/monkey do. A teenager's experiences can significantly shape their sexual orientation for life. Once that concrete has set a do over is impossible.
Homosexual men prefer cities because they can hide amongst large “communities” like the Castro in SF, and practice their “lifestyle” with a certain amount of anonymity, plus cities tend to be liberal politically. In small towns they stand out like sore, um, thumbs. As for causation, I’m not going there, but I don’t buy the whole mom/dad, recruitment thing. Too many homosexuals knew they were “different” from a very early age for external cultural or parenting factors to be important.