Well I have recently opened a porn and sex forum. I have worked on the forum and got it up and running with a smooth theme and section. I have purchased an ad-spot on another website. I am currently getting 1400 unique visitors a day, but only about 4-7 registrations a day, and not many active users and people that post. What am I doing wrong? Not many people want to register or post? The url is: www. xdiscussion . com
In my experience, you get a lot more posts if you do not require registration - but then you will have to keep an eye on it and filter spam a lot.
People normally tend to leave the site if they find the it not relevant to something they were looking for. As folks suggested, increase the no of post and informative content on your forum. This might help.
Maybe they are unable to type their posts as they squirm and get excited thinking about hot sex scenarios and leave their message not posted at all.
in my opinion you should open to public and do not require registration when you just start a forum. That will increase the post number. btw if do give people value on the forum they will actually register. try put some image ad, things you can give them.