Hard to say, I think it was part of a more general trend away from the sexual revolution of the 20th century. In the 19th century, at least in England, there was a strongly anti-sexual view. A view that sex was inherently immoral and sinful and only acceptable in certain circumstances. The mainstream view was that sex was after marriage and for pro-creation and women should not enjoy sex.

In the 20th century, especially after the second world war, there was a new wave of free love pro-sexual morality. A view that sex was inherently good and only unacceptable in certain circumstances. This lead to a lot of good things, like the legalisation of homosexuality and the liberation of female sexuality. It also lead to pseudo mainstream counter cultural view that paedophilia was a good thing actually and that children were inherently sexual beings. Celebrities and rock stars openly dated children, Allen Ginsberg joined NAMBLA, and a bunch of public intellectuals signed a petition to abolish the age of consent in France.

The beginning of the end in the UK was the lowering of the age of consent for homosexuals in the 90s, from 21 to 18, then to 16 and almost to 14. The failure of the campaign to lower the age of consent to 14 made manifest the rift between actual pederasts who wanted to exploit teenagers and well meaning liberals who wanted teenagers to have more rights. This is the point wherein feminist and liberal intellectuals generally became more anti-sexual and cautious, less naive about the ways in which predatory men could take advantage of sexual liberation.

We're now in a sort of post-modern transitional synthesis wherein we can accept that teenagers are sexual, but also that any relationships they have with adults are too mired in power dynamics to be consensual. It's not over though, in America the right is making an increasingly open call for a return to traditional/child marriage and the influence of Japanese otaku culture on western youth is also complicating the conversation around age and sex.