I wouldn't call that a standard practice, but I don't think there's anything particularly shady about Nutaku apart from the billing structure being connected to...what is it, Brazzers? There's too much for them to lose to make all this effort to build relationships over years with game developers and throw it away on credit card fraud. And honestly, I've seen more phishing and virus attack attempts from some major news site advertisers than I've seen from actual porn sites. (Porn site advertisers, on the other hand....)

In any event, it's your finances to worry about and therefore your decision. I'm not giving them any money because I can't afford to, and I only play those games of theirs that can be fully enjoyed penniless. But if I were in both a position and mood to do it, I wouldn't worry about Nutaku.

I always use an online wallet capable of issuing virtual visa debit cards, both to limit my spendings (as I have to go through extra step of stocking up my wallet before I can wast the money online, so I can have extra time to reconsider) and to protect my actual info. If it is not something you can readily find in your country, there's always prepaid visa cards... XD

I bought gold once with a prepaid visa to be on the safe side.