Either way, pinterest clone could be a very good thing for you to do (not just adult, it would work for other niches too). Pintastic is the top one, but they prices and terms are not good. Something top level, at least $1000+ as far as pricing. This would be doing good...
Pintastic 4 - the best pin script on the market
Something in the direction of sex.com. Make sure sign up is easy, social media sharing is good, words / keywords in urls (require people to add at least 5 words to every pin, lets say), ads can be easily added to the site. Something like this could have a tube within it (+ in-video ads). This would be making a lot of $$$, probably (for people working on these sites). Also mobile friendly, 100%.
Make something good, undercut them, it could / would sell better than the tube (again, not just for xxx sites, all types of sites).
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ImageZog Free Porn Pics - this is 3.5, I think. If they want upgrade to the newer version (they make one every 12 months) they would want them to pay like $700 for it, I think. This may be negotiated with them, before buying, but I am not sure.