Its a very good question and one that SHOULD be discussed.
Look at the overall picture, first Visa deams adult high risk, then paypal bails, yahoo! with them. The payment options are thinning out here.
There are still some solutions like dialers, checks, tokens, 900 numbers etc but you and I both know US based customers shy from those.
Another thing to throw into the discussion ... do you realize how many sponsors do NOT pay you on alternative join methods?? The single most common payout method for webmasters is when the surfer joins via CC. Don't believe me?? ask em .. I did - or heck i'll make it easy on ya ... click my tag and scroll down the column near the right named 'pay all join types' ... surprising huh?
well now take the CC away and whatcha got left?
How many sponsors tell you what payment options they offer to foreign traffic? How many even ACCEPT foreign joins?
yep, I can see the need for think tanks and discussions lol
heres a link telling more about the high risk of processing for adult
http://www.heraldnet.com/Stories/03/5/11/16917772.cfm