You know, in my headcanon, the facehugger isn't just meant for humanoids. We perceive it like that because the mouth is the most convenient orifice in which to get its proboscis into humans, but on larger animals, it could attack them like a mosquito going in through the skin.
So, I wouldn't suggest anyone turn their backside to one, either. Face-huggers be like, "A hole is a hole, bro."