I really don't think it's about anal sex FGS! Even people who do anal sex don't tend to do it exclusively. I don't think it's about getting a house either - I expect that kind of planning actually still happens among very deprived communities. Because it wasn't that easy in the 90s either, it was always a myth. (Granted, SH lists are much longer now, but I heard girls planning this less than 5, 10 years ago, when they were already longer than a long thing.)
It probably goes with the general shift in teenage behaviour - less rebellion and risky behaviour, drinking, drugs, sex, fighting, and more time spent studying and volunteering - teenagers have changed across the board.
Plus reliable contraceptives where the user doesn't have to consistently remember to use them, which is something teenagers tend to find difficult due to their immature brain development.
But honestly why have teenagers changed? I think it's parenting. Parenting has changed beyond recognition in a generation. It's no longer about demanding obedience and respect with threats, speak when you're spoken to, hierarchical family model with adults on top and children (and teens considered children) below, smacking has been frowned upon for 20+ years, it's encouraged to talk to your children, listen to them, reason with them and generally treat them as people with the respect that comes with that - and I think this has made a huge difference.
Despite porn handwringing (and I agree I do have concerns) - boys are much more likely to be taught about consent and respecting girls' bodies. Girls are much more likely to be told that they have the absolute right to say no, rather than being told not to "lead a boy on" etc. Of course this message does not get through to all but things are changing, and we should be proud of that.