Is it a requirement for a pay site to have a warning page?
 

I've been looking around the web and I've noticed that 1/2 the pay sites have warning/splash pages, while the other half don't. Is there a legal reason to have an entrance page, or is it sufficient to take the surfer to the tour page and have a generic disclaimer at the top?

If you dont have a domain with the word "sex" or "porn" or similar included you should use warning pages. Yes there is a law about it.