Not all ISPs will host adult sites for one thing. Some ISPs don't even offer hosting services. What is your ISPs URL and we'll look to see if you have to start this up with them thru an online form or something.
ISPs are dialup providers that usually host domains. Just like the one I am on right now. They host domains but won't touch adult domains.
There is a difference between like webair and say mindspring or AOL (as well as hundreds of small dialup ISPs)