It's all up to opinion. I personally don't think it's wrong if you don't watch it in exess; but others are totally against it and others still would say "fuck no, no problem"
It is true that you could be warned if you are caught by an officer. I highly doubt that you'd be arrested, unless you subscribed to a site, or you are interacting with people who run the site or you talk with someone that you've met through something that is 18+ related. Although, those people would probably be at fault because you are the one who is a minor.
I really wouldn't worry about it. A the very least, don't subscribe to anything, don't go in any chatrooms, don't get caught by your parents and heaven forbid, don't go to porn sites at school