Mr Jack writes:
I think it's quite obvious that the penis and vagina are co-evolved for sexual reproduction, whereas as the anus and penis aren't - unless you have some kind of evolutionary explanation as to what selective advantage is conferred by such a co-evolution?
The anus could have evolved a secondary function as an alternative way for couples to have penetrative sex. For use during pregnancy and when there were already a plenitude of young children in the family/tribe. Nature's contraceptive. That's speculation, I know, but it might explain why penetration of the anus can be enjoyed by the recipient as well as the penetrator.
Looked at that way, it might explain how the vagina would be the easy fit first choice, but the anus (and mouth) viable seconds.
If same sex relationships played a role, bonobo-like, in our ancestors, and sexual bonding between males helped a group to survive, then the "pleasure feeling" anus would also be selected for due to that reason.
I agree with you that "natural" vs "unnatural" arguments are fairly pointless. It is actually natural to our species to invent and make tools, and to build up sophisticated high tech. cultures, in the broad sense of the word, otherwise we wouldn't have done it.
In the narrow sense of the word, meaning perhaps our biological nature stripped of culture, I think that both the mouth (lips particularly) and the anus probably do have evolved characteristics that are directly related to sex, as do breasts (obviously), so if people are obsessed with what's "natural", they should just go ahead and use whatever comes naturally to them!
Fingers, on the other hand, are very useful in sex, but it seems unlikely that they've needed to evolve in respect to it. We could speculate about the extra length of the middle one if we wanted to.
In all cases they're secondary functions, and the penis and vagina could be said to differ in that respect, because their primary functions are reproductive. We could have a pissing system without them.
The whole system provides quite a good argument against intelligent design. Who would put a sewer in the middle of a pleasure garden?