I don't think so. If it's a large site with a lot of demand, then I believe that that sort of general experience would only help you at a later date. I'd say, as long as the company isn't being black balled in public forums, then you shouldn't worry about listing it in your resume.

I worked at a porn site a little while ago, and it was remarkable how much traffic we had to deal with. I left because I wasn't comfortable with working at a porn site, among other things (and funnily enough, they never told me that their main product was a porn site when I was interviewing for the position.)