Netbilling and your merchant account are two seperate things.
You can go get your own merchant account, and connect it to netbilling if you want. (assuming of course netbilling supports their means of whatever-the-hell.
I know we have a client who went out on their own and found a high-volume merchant account for their adult stuff (offshore I believe, but we're talking $400-500k/mo in volume), and are in the process right now of getting their sites connected to netbilling for processing.
We also use netbilling and CSI for our hosting operation. At another company we're involved in, we have netbilling and another (non-adult) merchant provider.
Other options are available for non-adult. Adult may pose much more of a problem, especially if you're looking for (lower discount rate) a domestic bank. If you look offshore (and also pay a higher discount rate) you can probably find something that fits your needs as well.
-Phil