OK, you asked, so here we go:
95% of straight men orgasm every time they have sex, compared with 64% of women.
That gap is due to a number of factors, but largely because the female and male bodies operate completely differently in sexual terms – from arousal all the way to climax and (obviously) beyond.
At the same time, largely due to lack of education and social / sexual equity, both men and women behave as though sex acts are comparable, and that women 'should' be orgasming just as much as men through the same acts.
So women are consistently not getting the amount or kind of foreplay they need to be able to reach orgasm, because they're expected to be operating like a man does.
That means not only are they not reaching orgasm nearly as much, they're also far more likely to experience pain during sex, discomfort, dryness, self-doubt in the bedroom, and eventually a generalised lack of libido.
Of all sex acts, cunnilingus is the most effective at preparing the female body for penetration, generating lubrication, and bringing a woman to orgasm – 30% more effective, in fact, than partner masturbation / manual stimulation.
It's the biggest tool at our disposal for closing the orgasm gap, minimising pain and discomfort during intercourse, and giving women access to a completely new level of sexuality and expression – proving that we're just as, if not more, sexual than men – we just haven't been having sex in the way that women's bodies are meant to.
If it's genuinely of interest to you, there's a book called 'She Comes First' which brings this whole topic to life brilliantly. My friend swears she can tell when a man's read it as soon as they hit the bedroom.
Anal sex, on the other hand, has no effect on a man's ability to relax in heterosexual sex, no impact on his likelihood of orgasm, no role in moving toward sexual equity, and certainly does none of those things for the female partner. It serves no purpose in a heterosexual relationship other than novelty and adventure. Which is fine, but it's nothing on what cunnilingus is able to create and make possible in erotic life.
That's how it's incomparable.
It's not about 'commonplace', or about where things are 'meant' to go – it's about one thing that has the capacity to make a massive impact on someone's sexual and erotic experience, and another thing that just... doesn't.