That is frustrating. I'm looking at my end and there isn't really any credit card setting or anything indicating how credit cards are accepted. Since the product is "flagged" as adult content, it could be contributing to the problem, but I can't see anything to do about it. 

I will be looking into new distribution because it's not just you having the problem.

The helps section is pretty lame because it puts all the effort on you, so I apologize, but I am at a loss. If you feel like trying the Blocked by Gumroad's risk system, let me know how it goes. I want to figure out what's happening.