Nope, humans definitely aren't the only species that does. I know for certain that Bonobos do, I'm almost certain that several other of the primates at least occasionally do as well. In terms of the anatomy, though, I think my point about extra padding applies in either position -- it's simply harder (both because of the padding, and because of the hip changes) for us to 'present' our vulvas in the same ways that primates are able to. I think that'd be easier to explain if I could draw out what I mean, but if you picture a profile of another primate or a human in the same positions, it seems to be that in almost any position, the human vulva is a bit more recessed between the thighs and buttocks (how much would vary hugely by body build, but I think the basic point stands).