As a webmaster and a father, I have always steered way around the adult industry. That said, I would be a hypocrite to say you should not accept this site as a Certified Partner just because I personally would not promote them.
Adults should be free to visit and play at that site if they choose to. Therefore, IMHO, that site and their customers deserve to same opportunities as other programs… especially if they pass your “due dillegence”.
Their is a market for that type of site. They meet the need of that market in a professional and upstanding way. I say let them in. 😎
==== EDIT =====
Looking more closely, I cannot find any actual gambling on the site either. If they are not a gambling site, then perhaps they are just looking for some sort of “credibility” to post a badge on their page or something.
Tough call…