You should promote it through adult forums & blog, also I assume there are few adult social networking site would be there that will meet your purpose.

I think you should use the relevant site link in your signature, however it is nothing bad if you use other site as atleast you can get some exposure.

I think you should narrow down your targeting to specific community, demography, culture and specific age to your niche area, it will help you to focus properly.

Well, for banner ad most important thing where you are going to place it and relevancy of particular site.