If you have lots of UGC, then AdSense is probably not for you. A lot of companies shy away from UGC specifically because it can risk advertising revenue from higher end brands that will pay more.

Fortunately, there are many different ad networks for many different publishers. Reddit is using Adzerk ... maybe they will work for you. There's also a bunch of ad networks that specialize in different areas, including porn, gambling, etc -- stuff that Google won't ever touch. You may still be able to use AdSense if you segment where the ad is shown in places where you're less likely to have negative UGC, such as non-UGC portions of your site or forum topics that are more moderated.