Not that I'm condoning anyone who's aggressively sexualizing the show, nor am I fond of bronies (never watched the show, but have seen their internet bumblings and the documentary) but I think there's a huge difference in fandom culture here.
I don't think I've ever really seen many people who are insecure about loving this show outside of the age group- and I can't help but suspect that it's because our demographic isn't stereotyped as 20+ men. There's a huge range of fans out there, and it's hard to pin down a societal expectation on them. I guess what I'm trying to say is that our culture doesn't rag on teens and women so much for liking something so... feminine? And by feminine, I mean it in the way that our culture defines it. I don't think characteristics like compassion and friendship are actually divided by gender, it's just that as a culture we make that distinction. Anyway, what I'm trying to say is that it's harder to guilt a mixed demographic.
With less guilt comes less backlash. I think the supposed aggression towards the mlp fandom created a sort of defensive culture that felt the need to unite and take ownership of the show. I'm not saying it's right, I'm just saying that there's enough people in the Steven Universe fandom to cultivate that issue... IF we were being harassed.
There are some blatantly adult themes here. I mean, look at those fusion dances. It's not just the Pearl thing. Amethyst pretty much dived into Garnet's crotch. Speaking of Amethyst, it is strongly implied that she started coming onto Greg as soon as Steven was born- in Rose's body! And Greg... did you see him pull that sci-fi erotica out of his book collection before giving it to Steven?
The truth is, I think like we're in the midst of cultural transition where cartoons are simply not confined to a single age group- and the writers seem to know it. I can't find the post at the moment (someone else MUST know where it is) but someone has approached one of the writers about risque art, and he essentially said it was never as lewd as their writing room discussions- and quite frankly he didn't mind it so long as it was properly tagged.
I am not fond of the idea of children finding inappropriate material online, but honestly I don't think it's that much of an issue in this fandom. It's not that common and handled pretty carefully. There's simply not enough tension for drama here.
...and really, you shouldn't be letting a little kid online without supervision if you don't want them running into adult content. They don't need to be looking for it to find it.