OK, so I'm basically sympathetic to this position, but we have to recognize that when we create a positive law like this, it's tricky. Part of what makes a good positive law is a clear, obvious, simple line. As an example: 18 (or any specific number) as age of consent. Why is it good? It's clear, obvious, and simple. As much as libertarians do the "BUT WHAT ABOUT 17 years and 364 DAYS" cope, the fact is, we have decided upon a line as a society and thus we (somewhat arbitrarily - one could make arguments for 20, or 21 as more acceptable!) have decided 18 years is that line. "Age ain't nothing but a number" - yes, but you have to abide by the law you perverts.
I don't know what the "clear simple and obvious" here is. After all, as you note, there's nothing inherently wrong with wearing school uniforms for sexual acts among two consenting adults. Similarly, there's nothing inherently right about a 40 year old having sex with an 18 year old. However, in the latter, it's a clear legal issue (disgusting morality aside). Partner over 18, not illegal, partner under 18, gulag. While I do like the addition of "big platforms" here, I just don't know what the proper line looks like. Are clothes that seem schoolgirl-ish unacceptable (so aping a uniform but not actually using a uniform)? It's just hard to police, legally speaking.
I want to make it totally clear: I don't think that "over 18" magically absolves people of immoral behavior. Power dynamics and age dynamics are a thing, but we can't as a society police this stuff (as much as I'd love to). Similarly, I don't think that "barely legal" material is somehow worth defending in the particular, rather, understanding what an effective ban looks like matters here. Thus, we have to use a clear demarcation that is arbitrary and brightly colored. I don't know what that looks like w/r/t the content of pornography, beyond the participation of underage parties being illegal/no snuff films/no scenarios that claim to represent illegal acts (so here, you can't "say" the actress is 16 even when actually of age - which remains a very clear and obvious line).
Final postscript: this is also not a defense of nonce island's legislation, I think we all agree those people are not acting in good faith.
13 is depraved.
I know some states have 16 as age of consent but I really think some push towards 18 as a global standard would do a lot of good. It's not perfect but a 16 year old is a lot more vulnerable since they're still a minor under the law. It doesn't magically make you an "adult" at 18 but having the protections of adulthood, such as they are I think is an important legal distinction.
The US has similar laws to what you describe as well (Romeo and Juliet laws) so that if two underage ppl (16 and 17 for instance) don't become criminal when one turns 18. I should say it's actually important to have those as well, because otherwise parents can use the law against their kids if they don't approve of who the date (eg. a gay kid who is 16 dates a 17 year old and after the partner turns 18 the parents could prosecute the partner w/o these carve outs).
Importantly though, this doesn't protect some 23 year old creeping on underage girls, and rightly so.
Ultimately I think a standard 19 or 20 for all adulthood (drink, smoke, sex, vote, drugs) would make the most sense, but I recognize some people think a 16 year old is old enough to consent. I just think the power dynamics there are disgusting