I don't think that it's christian nationalism in this case. The state definitely has a strong, compelling interest in preventing the sexual exploitation of children (and hopefully of adults as well). I think that it's more a case that it's really hard to figure out how to deal with material that is 99% exploitative, and 1% made willingly by kids, without them being coerced by adults.

Because--and here's where it gets really uncomfortable for most people--kids are also sexual. They may be more or less aware and interested in sex, and may not understand the mechanics, but that shit is baked into your biology. Gay kids know they're gay at a very young age, and I knew I was straight--although I had no idea what 'straight' and 'gay' even meant by the time I was 7 (!!!). And this was well before the internet.