Roughly speaking, it looks to me like younger people do not want to get married, and are in fact choosing partnerships that are expected to be only semi permanent instead. The idea of traditional, permanent marriage is taking on stuffy and backward-thinking connotations.

Some will still get married, even forward thinking secular people. There's a big difference between trying to build a family with someone who wants to partner up for as long as they're not unhappy, and having kids with someone who at least aspires to a permanent commitment. Marriage is pretty messed up in the U.S., but I still think it makes more sense to get married if you want to have kids.

Some won't. I don't think a culture built around transitory relationships is a good one for either parents or children. I wouldn't be surprised if there is a backlash in time, though I think another possibility is people just give up on having kids altogether. We'll have to see.