I’ve no doubt but that there are more gays in cities, but I’d be curious as to by what process you believe cities make men gay.
They actively recruit in cities.
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.