In the view of Buddhism, the cause of homosexuality is related to one's constitution as well as from psychological conditions. 

From a religious viewpoints, homosexuality cannot escape the item: contradicting morality -- having abnormal sex.  However, from the Buddhist point of view of cause and effect, all worldly sexual desire among men and women, men and men, and women and women contain cause and conditions.  The value of the Buddha's teaching is that the Buddhas and bodhisattvas understand causality very well.

Since Buddhism teaches the existence of passed lifes and future lifes, human being has been reborned again and again in different lifes, and in different form, as if male or females.  In all the passed lifes, each human had made connection or shall I say having affinity with different people, and these affinity bring forward to the next life as the next episode of the story, the love affair or story continue to the next life, while the couple might be borned as the same gender, their affection toward each other from the previous life continue, ...

As a general, Buddhism teaches the existence of passed and next lifes, cause and effect, negative action give negative karma and bring unforturnate consequences to current or future lifes.  These could explain why there are people borned to be unfortunate and some are borned with all the welfare that they could get.  Life is always fair and equal. in Buddhism, there is always an explaination toward any circumstances.