I was just thinking in terms of attractiveness. Kids today have an unlimited supply of videos of beautiful people having sex. Not only are their expectations rising, but their choice of attractive mates is falling.

So they'd rather just watch porn than lower their expectations and have some sort of sex?

It's not so much that they'd rather watch porn. It's that porn sets their expectations regarding what sex is and with whom they expect to have sex.

If I'm a 4 and I won't settle for less than an 7 (which is a step down from what porn is teaching me about averages), it's likely I'm not going to father many children.

If you're a 14 year old boy wouldn't you go for pretty much anything you could get?

Yep, that was the case for me. But two things: One, it's not the boys who decide when sex happens, for the most part, and two, the amount of effort a boy (or a man) is willing to put into wooing a woman depends in some part on how attractive she is to him.

Do you think it relates with what's going in Japan?

I'm not parent but I wonder if it could be both? Though I think the former was the original intent but I hadn't thought about the latter.

My wild hypothesis is that teens may be more aware of the rising cost of parenthood and may rationally be avoiding the situation altogether.

Parenthood provides virtually no economic benefit in modern industrial society. More so now, with mobility and weakening of family ties. Children cost a lot, and they probably won't help when you're old. So why bother?

I think you underestimate the extent to which pregnancy was always considered an Unthinkably Bad Thing by teens. And also overestimate the extent to which teens are rational.