I've been playing here for over 13 years, and you can have full, meaty, vibrant games without so much as the Mature flag. Just take into account the attitude you're presenting: you want people to let you play games you can take seriously, but you aren't willing to take seriously any game that isn't prepared to include sex-porn or gore-porn. I've never seen any published module for any RPG I've played that would ever require an Adult flag, and the only one I played that would require a Mature flag was a World of Darkness adventure with a drug trade involved.
Really, just give the games you're allowed into a chance. I guarantee you'll only get super childish games if you literally go looking for them. Just be aware that your complaint sounds pretty childish on its own. Trust me when I say you aren't missing anything you're asking for.
Swordmasterg:
It also says crime and gore, which means I'm just banned from using some D&D monsters, like the Gibbering Mouther(Which would fall into the gore category).
Gibbering Mouthers aren't gore, and the flag is for explicit, gratuitous gore specifically. Gibbering Mouthers are just mounds of flesh with mouths that never stop babbling. You don't need a Mature/Adult flag for that, unless you decide that yours in particular are specifically gore monsters, and you decide to describe them in sickening detail.
Heck, you include sex in games that aren't flagged Mature/Adult, long as you don't show it. Or include children, because even the Adult flag won't save you there.