Here's my theory, totally unsupported by any scientific evidence whatsoever. The Human Being's default sexual state is bisexual (it's no coincidence that our closest natural cousin is almost exclusively bisexual). However some will have a preference to one side or the other. Be this preference genetic (as supported by this research) or developed over time (as supported by the number of blokes who are repelled by the idea of same sex union, but can't quite put their finger on WHY).

I am prepared to put this nature or nurture argumemnt to bed by giving what I feel is the most likely answer. A BIT OF BOTH!