Google

Porn sites looking to advertise with Google will need to find other places to go.

The search giant put a new policy into place on Monday, which bans the promotion of most sexually-themed sites from its ad network. The change goes after sites that advertise hardcore or "graphic depictions of sexual acts," including masturbation and oral sex. Google announced the policy change in March.

The ad network, called AdWords, is Google's platform for advertisers looking to place ads on both Google sites and on properties across the Web. Advertising is the company's cash cow. In the most recent quarter, Google made $13.8 billion in ad revenue, accounting for 90 percent of the company's total revenue.

A Google spokesperson declined to break down the percentage of how many sexually-explicit websites advertise on AdWords. When asked what prompted the change, the spokesperson only said the company updates the policy on a regular basis, and has had strict guidelines for a long time when it comes to sexually explicit content.

Some in the porn industry said they were blindsided by Google's announcement.

"I was one of the very first advertisers for AdWords back in 2002," Theo Sapoutzis, chairman and CEO of AVN Media Network, told CNBC. "It's something that's been [untouched] for 12 years, so you don't expect change is going to start happening."

This isn't the first time Google has reined in adult content on its properties. Last June, Google-owned Blogger banned porn blogs from making money off of ads for adult websites. While porn ads are now prohibited on AdWords, the updated policy still allows for ads promoting things like strip clubs and adult dating sites.

The search giant informed companies of the new policy changes in an email that was sent out in June, published earlier Wednesday by CNBC:

When we make this change, Google will disapprove all ads and sites that are identified as being in violation of our revised policy. Our system identified your account as potentially affected by this policy change. We ask that you make any necessary changes to your ads and sites to comply so that your campaigns can continue to run.

The move comes as Google continues to invest in other forms of advertising -- especially online video. Last month, Google announced that it acquired mDialog, a company that specializes in video advertising technology. The startup works with large media companies and lets them manage and deliver video advertising across several screens, including devices running Google's Android mobile operating system and on Apple's iPhones and iPads.