A population being 100% heterosexual is not natural for many species...?

There are two sexes. That doesn't mean that all or us or any of us are perfect examples of either. The way biology works, we all start off as females. For males, the sex organs move through the body wall and move to their final place. If hormones and lots of other things happen in a certain way, it makes us one way and if they happen in other ways it makes us in another way. There are a gazillion different possible combination of features. Anybody that pretends theirs is the best is probably just insecure.