Yeah have and still am, among others. Big market, big opportunities. Takes skills and capital though.
To be clear, to go the traditional route of buying ads to push traffic and offers, you need skills and capital.
If you truly innovate and create something great, you get a lot of marketing for free once you start getting your name out there.
Don't let the difficulty of pre-existing systems stand in your way. If you want to be a porn tycoon, maybe PPC or PPV ads aren't for you. Maybe, instead, you can offer a different kind of value to this industry.
But now that begs the questions: how much do you REALLY care about this industry? Enough to iterate over all of these failed ideas in turn?
Think about it.
What is your purpose?