Well, you can see Apple's side of this, no? You're basically saying that you don't deserve the mature, "internet app rating" because your users as usually pretty quick at flagging things or that you have a safe default to catch the rest (which can be turned off by creating a 500px account and just turning it off). Both of these subvert Apple's built in parental controls and I'm guessing that's exactly why this happened. If there's a chance of mature stuff, you have to set your rating correctly.

Everyone already got all bothered about this ages ago. Apps that had built in web views had to set a mature rating because a user could then surf the web even if Safari was disabled.

I guess this comes down to what they've asked (yet?) of you. Can you resubmit the app with a higher age-restiction?