Your options are VERY limited, which is why so few do it now via thier own sites and usualy do it via adultwork

First way is to get your own merchant account and handle things yourself, but as your going to be considered a high risk (read expensive to obtain) account, I cant see if being cost effective.

Second possible way, http://www.ccbill.com/ have been used by porn sites for as long as I can remember so maybe able to help either with you using thier E-ticket system say to buy access to an order form, or they may have another solution, again though I dont think your going to be talking cheap or easy set up but best bet would be to contact them.