I've had a profitable adult pay site for 15 years now. Since then I've branched out and created more paysites, with another one in the works.

Some people over the years have become very good with Adwords and can target traffic extremely well. For those people Adwords is a form of cheap targeted converting traffic.

There will always be someone between you and your online prospects, whether it be Google, an ISP, Social Media, etc. Google, ISP's, Social Media (for the most part) have all made it clear they don't want outright adult in peoples faces.

Spammers have totally destroyed it for legitimate business people in adult, well not destroyed it, but made it very difficult for us to be seen nowadays. A lot of spammers have no idea they've been relegated to virtual black holes on social media sites where they're not actually communicating with the masses. Some ISP's, and countries, block adult sites. The people in power have been dividing, and conquering, the internet for a while now, cutting it into little platforms with their own anti adult policies. The list goes on. To be very successful you need to come in clean, and stay clean.

It's best to have a property or two that's PG and serves the interest of your niche. Adwords will accept your site for advertising. You lose a lot less traffic filtering through that way. I know a couple people that do this, and it works very well for them.

Again, this is not for gaming the Adwords system in any way, or spamming, it's giving them what they want, on their terms, to get what you want. They want ads to be useful, varied, relevant and safe. They want sites not created primarily for traffic generation or gateway sites, so give it to them while serving the interest of your niche. If you aren't, or haven't been, interested enough in your niche to create something of value for them outside of porn then don't use adwords.

Successful long term mainstream promotion for adult means you are subtle, don't link (most of the time, on most platforms), and a list of other things. I call it passive promotion. I'm sure there's some other name for it but that's what I call it.