Sexual unions, especially for the male, is a tough, highly demanding exercise. In all forms of animal life, immediately after mating, the female can be observed as one with a highly dissatisfied look, and, in some cases, like certain breeds of spiders, with very violent disposition. Honey bee for example, maintains a whole army of worker bees, the only function they are required perform being to sexually entertain the queen. All male animals, aware of the impending disgrace after each mating might have been reluctant to enter into sexual union, forcing nature to come up with the idea of mating season when no other thought disturbs him.  I think our forefathers preempted such a fate if things are to follow a natural path and devised various ways to reduce the necessity to perform sexually, as far as man is concerned. Thus we have sex prohibited on various grounds, of certain relation, certain time, giving abstract meaning to gratification and it's results etc. Homosexuality is one such method to escape from the rigours of performance.