The fact is anal sex is already a normal part of sex for young girls.
I'm in my early 30s, I've spoken to girls only 10 ish years younger than me that told me it was normal to lose your 'bum vaginity' as a teen and in a casual relationship / one night stand. It's also normal to do on a one night stand.
That is not what it was like when I was a teen. The only girls that did it were 'dirty' girls, and/or those easily pressured. Girls engaged when older and in long term relationships.
This sort of casual joking about it is part of what is normalising it. And I don't think the teenage girls part of this culture shift are happy or thankful for it.
Oh and anal does increase the risk of transmitting hiv. That's why it never spread in the hetro community in the same way it did in the gay community.