At least the soy is plausible. Soy contains chemicals that mimic estrogen. That, in excess, can in fact tamper with development of your sexual identity. Other similar chemicals that can do the same thing are the bisphenols found in plastics, various pesticides, and quite a few different drugs (I would worry mostly about the psychiatric ones). Also suspect would be things like monosodium glutamate and aspartame in your food, fluoride in the water, and too much soy or canola oil and not enough coconut or palm (or natural fats like butter). And, we can't forget simple genetics--that can play a role in whether a person becomes homosexual.
Another reason is the hang-ups that religions create about sex. Exposing children to same sex children exclusively and dirtifying the mixing of sexes while children are young can shape a child's attitude. Also, religions that habitually put people of the same sex for a prolonged period on a project (the Pious-Sneer Assist Program was one such example) can enhance the chances of homosexuality, along with limiting the exposure to the opposite sex. When such religions also practice homophobia, they ought to be ashamed of themselves for this squeeze play.