The best paying I've seen so far is Infinity Ads, Clicksor and CPX Interactive. Clicksor's popunder ads are the worse I've seen, I hate the ones that ask you if you're sure you want to leave the page over and over.
CPX is probably the way to go...I've used them for non-adult sites. Clicksor pays badly and their ads really have issues with malware from what I've seen.
You can giving Infinityads a try. We payout every 15 days with only $5 minimum. Currently we accept traffic from US, CAN and UK. If you want more info, you can visit our site. Thank you. Best, Infinityads
if you are more cared about your visitor then better avoid popunder or popup ads, which really hurts them on coming to your site. If you use then don't pop a new window on every page view. for example, if i visit 10 pages on your site, it pops 10 windows and believe me, the next time i won't come to your site because i have to close 10 windows If you really want to utilize every visitor, best alternative i could think is banner ads based on the impressions which well suites for highly traffic sites. Or simply sell Ad space on buysellads or oiopublisher !
Well, what if you get only 1 a day? Wouldn't you not mind it? From my experience, even putting 1 a day pop unders / pop ups worked out fine and didn't lose any traffic.