Black label is a good alternative for adult, plus you can check what ads are clicked, which will help you go to the next level...
The next level is using a text ads zone looking like AdSense or BlackLabel, but going only to your affiliate programs. You can try a lot of different niches, as long as the random factor makes believe they're regular ads with title and text.
In the meantime, as your forum is now catering to adult content, you can try other revenue sources, such as
Ashley Madison, a neat dating site paying up to $200 per membership (from the first member, I'm not talking here of having 300 signups a day to get that high return), and supporting PayPal among other payment systems.