It's easy and I don't get why it isn't common. I see people on forums all the time saying "I wonder how suchandsuch would do", it isn't hard to set up a site (pickyourporn.com sexyourway.com, whatever)where the content is "voted in" by your surfers, you can set up polls, mini quizes, forums, and a number of other things to create an interactive adult site where your surfers decide what they want, and what they don't want, and what about it turns them on/off. You can also set up "feedback" 15 pic galleries where viewers can post what they think of the set, how they would have done it differently, rate 1 to 10, etc. Not only does this give you a clue what people think of the content you already have, but what you should be getting more of. If you have a site like this, you'll learn a LOT and be able to use the knowledge to benefit all of your other sites. Stop wondering what surfers want to see and ask them.
By the way, test market in ALL the niches you think might work, people with specific tastes usually have even more specific tastes that aren't being fulfilled online.