It is entirely plausible (though admittedly unlikely) that via natural selection humans have "evolved" specifically that our anus ends up being roughly the same size as a vagina.
Pre-modern birth control couples would need to abstain from sex, perform oral only or risk impregnation. Having an extra "hole" would allow couples to partake in sex without the risk of someone becoming pregnant when they didn't want to.
Using a Darwinian model individuals would be selected for mating based on the size of the anus, with individuals that had a smaller anus being less desirable and therefore less likely to spawn new generations of humans with similarly small anuses.
Not only is this linguistic dynamite one of the most outstanding things I've had the pleasure of reading, it's also entirely plausible.