While I'm sure there are some hosts who make that decision for "moral reasons," most that I've seen don't say that's the reason -- most blame it on the resources used by the typical porn site. It seems like it'd make sense that their bandwidth policy would cover that, but there's one reason it doesn't: most hosts are overselling both their bandwidth and their disk space.
For the most part, the average web site doesn't come close to using all of the disk space the account is allocated, and hosts know that. But a site made up of images and video for download almost certainly will. Same principals apply to bandwidth usage.
Add to that the fact that commercial adult/porn sites are frequent targets of spam complaints, and you have another reason why some hosts don't want to deal with it.
Certainly those things aren't true of all sites that contain adult content, but they're a common enough occurrence that some hosts won't accept it. Then too, many small hosts are constrained by the content rules of their datacenter and upstream bandwidth providers... and some just feel that they might lose other business from potential customers who don't want their own sites hosted at a place that hosts adult content, either for the customers' own "moral reasons" or because of a perception that they'll be innocent victims of hack attempts, DOS attacks, or just high resource use. Again, that might not always be the reality, but it's a common perception.
[edited by: JayC at 9:06 pm (utc) on Dec. 17, 2002]