Embedding videos is perfectly legal as long as porn is allowed in your target country.
If it's going to be just another tube site full of embedded videos and you are going to monetize only with ads then you need alot of traffic for dirty cheap price to make any real profit.
If that's the case then your best bet is organic search traffic (surprise). The trick with ranking a tube site is to put some effort to those videos. It's not going to work if you just use a mass embed script to rip the videos from big tube sites with copied titles and descriptions. Write your own titles and try to write long descriptions to every video with original content. Also make sure that the tube site you get the videos from is not too aggressive with their own advertisement because that's going to steal a lot of your visitors. For example red tube uses pop unders if you embed their video, pornhub is less aggressive.
