As your visitors are offered free porn content, and they come back for it (40% of them, impressive), you should offer them something else than paid porn (why would they pay for it?). Give them something real, a real-life experience. As you said, penis enhancers, dating sites, live cams, abs pills, sex toys... anything can (and should) be tested.
I'd go for the whole package, try everything at the same time and expose them a bit more with anything that converts after a while. You can even try out a pay-per-click program like
Black Label.
For the dating part, I suggest
Ashley Madison (up to $200 per sign-up, from the first one), and
Fling and MeetLocals (a flat $35 per sign-up).
And for the cam part,
Livebucks ($35 per join, up to $100 depending on the traffic).
Experiment a bit of everything, that's the only way to detect what will convert your audience.