I guess, from a species point of view, the whole point is heterosexual attraction is the one that makes the babies, so with that reasoning, it's more about why are we sometimes attracted to anything other than this rather than asking why we are sometimes restricted to this. I don't think there is any easy or definite reason why people are attracted to the same sex which isn't helped by the fact that some explanations can be perceived as offensive by those to whom it applies. Even if attraction to the same sex is caused by (and I don't mean this offensively) a biological accident that causes attraction in the "wrong" direction, there might be some evolutionary benefits to this, such as aiding bonding through affection and emotion. I have thought at times that while people find others of the opposite sex aesthetically attractive for reproductive reasons, it is also normal to find members of the same sex aesthetically attractive (with or without sexual attraction) in order to gauge the attractiveness of competition for their chosen partner. In any case, I don't know nearly enough about biology to really formulate any theories but I've also thought that there must be (at least) two different mechanisms in human kind- one to detect male pheromones in order to respond sexually to them (in the case of females) and the other vice versa... The existence of bi/pansexuals seems to indicate that either we all have both of these, only one is "switched off" in monosexual people, or otherwise that only bi/pansexuals have both, which I think is less likely. In my case of course, they are both "switched off". Maybe in bi/pans with a bias, it just means that one of these is better at perceiving the pheramones than the other or something, I don't know. And with all that waffle, I'll stop...