Monks in general have lower strength than fighter types, since they have to spread their abilities out a lot more. Grappling is based on base attack bonus, so fighter types get the best of monks there, too. At lower levels, a fighter type will have at least one more strength bonus and one more BAB than an equivalent monk, so the monk will only be up by 2 to his grapple check. That's hardly unbeatable. At higher level, the fighter's BAB is going to quickly make up for the monk's improved grapple.
Wizards and Sorcerers should be smart enough to stay the heck away from hand to hand combat. If they're getting caught by a grappler, it's their own damn fault.
And all these guys should have some allies, dammit. Sure, the monk can grapple one melee combatant, and have a slight edge over him, but where are all that guy's buddies to do to the monk exactly what the monk's allies are doing to that guy?
It's really not as bad as you think... you just have to make sure your NPCs fight intelligently.
-The Souljourner