I just think that's there's this message that anal is actually really great, if only you'd give it a chance.
And actually, it's not just not great, it's downright painful - no matter what - for a significant number of women.
There's an expectation of it now amongst many men, as @BlancheM says.
And the upshot is that many young women are being pressured into doing something many of them don't feel able to say no to.
This thread is an example of this - 'you're not doing it right', 'if you just do X, Y and/or Z, it's great', 'with the right man, it can be fabulous', yada, yada.
And that's all well and good for those of us confident enough to say, no, actually it's not for me. Not so great for those who who don't feel able to articulate it.