Especially because of this sex discussion, I'm leaning more towards:
Cantrians are quite, but not completely similar to humans, especially when it comes to, well, procreation. In short: Cantrians are not born, so there's no need for sex. And to take this a little further: there is even no knowledge of sex.
That's the way I decided to go with my characters. If that's ok or not is questionable, of course, but beside the point, at least in this thread.
The point here is why I decided to go that way:
The concept of indulging in "written sex", be it as in RPed sex in Cantr or "cybering" in a chatroom is completely and utterly lost on me.
I like sex in real life

That's why I'd like something like a flag that says "No Cantr sex please", as it was suggested somewhere around here. When I take a look at the poll, there might be quite some more people who'd be interested in something like that

As for the legal aspects of this matter, as mainly Seko and Sunni pointed out, to be honest I'd try to see to it that either sex or minors are banned from the game, just to be on the - more or less - save side.
(Guess what I'd rather see banned

I'd not want to be in Jos' or the PD's shoes if some parent does overreact when seeing their children doing "dirty stuff" in some online RP game.
On the other hand, Seko has a point there: seeing animals have sex or being in close proximity when parents have sex or even getting married at 12 was common, at some time in the past or another. Maybe our time is a bit... tense when it comes to handling this matter (and too open with it on the other side - just try to count ads that use the famous "sex sells" method for one afternoon, be it on posters or on TV)
(*) (RPing, let's say, the buildup of sexual tension in a pen and paper game with acutal people sitting around a gaming table is yet another matter - given that I know the players well enough this can be quite interesting. So, the text form seems to be the problem, not the RPing.)