Perhaps is part of human evolution . Since most human penetration into vagina is when body are face to face ( or on on tot of the other ) animals penetration is front to back.
Okay, look ... um ... I don't know how much you actually look at men, but that just has to do with a matter of detail. So, XY, describing males, is a genotype. The outward appearance of a male―no significant mammary tissue, lack of vagina, and presence of a penis, is the normal phenotype for XY. XX, describing females, however, is the natural default; it's what humans are. As such, as an XY blastocyst develops into a fetus, the first apparent phenotype of an XY human being is female. Sexual development. There is, though I forget the detail as I learned it over twenty years ago, an obscure episode in the anthropological and medical literature about a handful of strange cases in the Caribbean when apparent young girls (phenotype XY) approached puberty and began a shocking transformation into males. And in more recent years―over a decade ago, though―I happened across some documentary series on human sexuality featuring interviews with a former collegiate volleyball player whose biology professor helped her figure out and prove that she was, in fact, genotypically XY; for whatever reason, she had reached adulthood without the transformation occurring. It happened to all of us human males, though generally speaking it occurs in utero; as I understand it, the developing organism receives a hormonal dose from mother that spurs phenotypical development of XY. And until then, the gestating XY looks just like gestating XX. At any rate, next time you happen to witness a naked male somewhere―or, I suppose, you could check yourself in a mirror―you will notice a line in the flesh running along the length of the prostate. That's where your vagina is supposed to be; I forget if it never opens in utero or seals at transformation. And his penis? It was, until his body undertook its phenotypical transformation, his clitoris. (I don't know the last time you took some manner of impact to your testicles, but, you know, taking one in the nuts hurts, and pretty much every guy I know agrees that it hurts up in our abdomens. There is a reason why it hurts there when our testicles experience trauma; that's where they come from―that's where the connection runs.) If we borrow the notion of interchangeable parts, a penis is just a naturally-mutated clitoris.
