to my understanding, gay i.e. The preference for same sex is the result of nature. However, bisexual is nurtured.
Nature makes sexual acts a pleasure so as to encourage procreation. As a result, the desire for "pleasure" is the primary reason for animals to engage in sexual acts and not the need to reproduce. When young male lions were chased out of their pride when they reach maturity, these young males engage in homosexual acts among themselves as the urge is great but there is no female around. (our local zoo has only 2 male lions and once i witness one mounting the other.) however, once the young males managed to take over a pride of females, the homosexual activity will stop. An important point to note is that their earlier sexual acts have greatly strengthen the bond between the males. They have known each other intimately.
For human, if a very young straight guy meets a gay, he may not be able to resist the "pleasure" performed by the gay on him and engaged in his first homosexual act. Since male partners are not easy to come by, the gay will cling to him and the bond will grow. Though the straight guy may still find the opposite sex attractive, it will be difficult for him to break the bond with the gay. To many social workers, this is the main reason why they have an issue with gays. Gays are born but they can make straight guys bonded to them for life.