No it's not...infact, it didn't have ANY, nor did any other system before PS2, because Grand Theft Auto: San Andreas is the first and only game to recieve an "Adults Only" rating.
I can't tell if you're being sarcastic or speaking strictly from the ESRB rating system. But before the ESRB formed a system, Sega had long since been rating their own games for content. The ESRB just took that over

Really? Never knew about that... details plz?
I'll have to hunt down the old Sega Visions or Game Players magazine that had the article on adult games, but one that sticks out in my mind was called Brain Seed. But that was rated for adults only more because of its mature complexity and whatnot. And gore. But I don't think there was anything sexual in it. I dunno. I'll dig around and get back to you guys. But I know there were quite a handful of very mature games on SegaCD.