Societal training and drilling turns them str8. Only the valiant ones resist.
Animal kingdom uses M2F sex only for reproduction. The rest is masturbatory activity or M2M play, flirtation and downright M2M sex.
http://en.wikipedia.org/wiki/Homosexual_behavior_in_animals
At young age girls and boys abhor each other. That's normal. What's similar always appeals more, what's dissimilar always less. They get duly trained and drilled the most shameless ways (culture, Hollywood, school and all other usual bullshit) into str8ness.
Peer pressure is paramount. In order to maintain appeal to peers (nothing is more important for boys), they turn str8 so that they maintain all "manly" sex appeal for peers, though it is reformulated in "no-sex" compulsion and even sex repulsion. Appeal diappears when they are not in keeping with fascist str8 imagery. The reformulation of sex appeal into repulsion for each other can lead and leads to worse homophobia ever.
Sports are used as a safety valve. Male sports are male porn for the str8 male which is reformulated in "no-sex" compulsion. Sports are paramount in all societies in order to mitigate boy's castration feelings from young age. Moreover boys are raised into shame about sex in order to str8ten and castrate them better.