What materialism can't explain, at least to my philosophically and biologically untrained mind, is why any such behavior is good, or bad, from a moral perspective. All it can do is to show the norm, and that individual creature A behaves different from that norm; it can't explain whether the behavior is right or wrong.
Materialism might postulate that an aberrant behavior hurts the species' chances of survival (though such an argument might more easily be made in hindsight). But it can't tell us why extinction is bad.
It can't even tell us why death is bad, since when one thing dies, many other living things get to nourish themselves on the carcass. As the outlaw Josie Wales said, worms gotta eat too, same as people. Modern biology, as materialism's water carrier in the natural sciences, should be pleased either way with the outcome.
Humans detachedly observe in other species behaviors that they would condemn as immoral in other humans. When lions kill each other in territorial disputes, the behavior isn't called evil, it's just what lions do. When chimps eat a female from another clan, it may seem repulsive, but again, they're just doing what chimps do -- I've yet to hear a biologist refer to this behavior as "evil". In fact, letting her live might raise the biologist's eyebrow, were that actually the aberrant behavior.
So it seems to me that, if we're discussing any morality derived from biology, we need simply to understand:
1. When humans act in certain ways, they're just doing what humans do.
2. Since they're just doing what humans do, there's no right or wrong, it just is.
3. If a behavior renders us extinct, that's okay, worms and buzzards have to eat too.
4. Nonetheless, there are behaviors that we like and don't like.
What this leaves us with is preferences -- morality minus authority. E.g., if gay men like being with other men, that's natural. But that's a two-edged sword: if straight men don't like gay men's behavior, that's natural too. It all boils down to what we like and what we don't like.
If morality is biological, I'm afraid this is the world we live in: morality is but an illusion, alive so long as humans are here to uphold it, dead and gone when the last human is.