If you're one of Pornhub's 60 million daily visitors, you might be wondering why you saw ads this month for clothing brand Diesel rather than, say, male enhancement pills or hot singles in your area.

It was certainly no media-buying accident. In fact, the fashion brand bought prominent placement on the site from January through Valentine's Day—and it's likely that more mainstream brands will follow suit.

"We wanted to have a relevant digital campaign. So where do people go online? They like cute cats and porn," said Alban Adam, Diesel's PR manager for Northern Europe. "We said, 'We need to be there because that's where the traffic is and let's not pretend that nobody goes there.'"

According to Richard Welch, global head of strategy for Spring Studios, the creative shop behind the campaign, advertising on a Pornhub makes sense for the Diesel brand.

"There's a relevance here; this is intimates, underwear," said Welch. "We're not advertising accessories, we're advertising underwear, and Diesel is a brand that has never strayed away from sexuality or sexual behavior."

While Diesel isn't the first mainstream brand to capitalize on Pornhub's vast audience, it is the "biggest brand" and the "first fashion brand," said Corey Price, vp of Pornhub.

"There has been a heightened interest in the last few years, from mainstream brands who want to partner with us because of our large global reach and visibility," said Price. "We anticipate more and more brands reaching out and inquiring about advertising with us as we continue to gain additional visibility."

Brands looking to advertise on Pornhub and its various sites aren't just doing so because they attract a large, diverse global audience but also because there has been a generational shift with regard to the taboo nature of watching porn. "Younger consumers, the reality is, they have access to pornography 24/7," said Welch.

Price agreed. "Truth is, we have a tremendously devoted and engaging user base, which is ideal for advertising brands. However, brands don't know this—or they think it's too risky to partner with an online adult entertainment company. Marketing moves like this really put brands like Diesel ahead of the game."

The partnership helps Pornhub, too, as it continues to work to remove the stigma associated with watching porn online. The brand has done so by getting into the music business (with Pornhub Records), the fashion business (Pornhub Apparel) and even philanthropic pursuits (Pornhub Cares).

"Partnerships like this, with a household name, lend credibility to our brand, increases consumer awareness and can help create additional partnerships and collaborations," Price said. "We have worked hard to make Pornhub visible in some unexpected ways, and to appeal to all kinds of different people. Pornhub is becoming more and more palatable as we continue to diversify in our goals, partnerships and overall audience."