I think a big part of it is that being a "prude" isn't as big a deal as in prior generations, but girls in particular are more empowered and can be far choosier in whether to have sex, and with whom, without losing social capital. I think it goes along with trends in which the younger generations don't binge drink or do hard drugs as much as older generations did at their age, and the social pressures to do those things is far less. I've seen it with my niece who is now in her early 20s and my daughter who is in middle school. I also think the internet phenomenon isn't just about online porn, but how gross dudes can be in dm's,etc. As a result, young girls either lose interest in being sexually active that early, or are far choosier. An article that was posted here (I think, or I saw it elsewhere) talked about how girls in Gen Z and Gen Alpha often wear socks with sandals because of all the gross requests for feet pics they get if they are barefoot or in sandals. I see some correlation on that topic.
TLDR: Dudes are gross, girls are finally catching on.