You mean china mainland, not the more free taiwan or hong kong eh?
If such a forum existed, it would be blocked by chinese firewall:
https://en.greatfire.org/blocked-websites
Even if they accessed one with vpn, it would be monitored by chinese police to bust everyone involved. exmaples:
http://www.echinacities.com/aroundto...n-website.html
http://tech.sina.com.cn/i/2012-07-30/05397442698.shtml
A few chinese adult sites exists but they change name and ip weekly so not even worth to post there. all what they advertise it is life-size dolls (some are cute), and miracle pills and lotions to be superman in bed.