As long as you own the copyright or have the copyright holders permission to use their images/ videos and you ensure that the content you display meets your local laws on decency then they are perfectly legal to run. The challenges can be getting companies (eg webhosts) to be involved with you. Most don't want the association and those that do allow it tend to be massively over priced due to the lack of competition. Thats before you work out how to get legal content and actually get traffic in the most over saturated market on earth.
You need to have the licensed because those content are not able to access to all countries. Some government are not allow those content to enter their country.
It is a multi billion dollar industry but it is heavily saturated. If you have a great business idea of how you can get traffic to your site, have a source for legitimate content and have worked out how you will monetorise the site (adverts? subscription? something else?) then go for it. Even if you only got 0.001% of the market you could probably retire in a few years time. If your going to be one of the 10,000,000 sites that get 1 visitor a month with illegal content and no income then it probably isnt worth it.
Unless you're using fresh new content (eg. you have you own actors for the videos), then don't bother.
It would be difficult. It would be very rare to find an adult industry that "didn't know the internet is for porn." You could create a website for "amateur" content. In otherwords, a website produced by others. But I suspect a "youtube" for porn already exists. It it doesn't, I just made you quite a bit of money!
Craigslist is the solution to all problems! All hail Craigslist! No, seriously. I'm sure there are at least a few sites that would let you advertise for XXX material (that is original, or just sites that let you hire actors). After getting a small site up and running, you could probably just advertise on your site. (put a link saying something like "Do you make videos? Click here!" I'm sure you've also seen those advertisements that say "we'll pay for videos, just send them here and you'll receive money"
Not true at all. I use Dreamhost to host my adult website and don't have any problems at all. If you decide to open a site, use promo code MAZUK when you sign up for $50 off the first year of DreamHost.