It's an advertisement for viewers to "see more" -- there's lots of free porn out there, and most of it is used to funnel a percentage of the viewers to paid subscription sites where other / more similar content exists. Conversion rates are not as low as you think.
Expanding on rexreed's point, these are people peddling a vice with the characteristic of inhibiting the frontal cortex and tapping into the ancient hindbrain of the human brain. They can win statistically just by exposing you. Of course the human forebrain is not "helpless" against this assault, but it's still a game they can win on the margins. (I find it an effective defense to be offended that they would reach for that; this is also a good cognitive defense against "won't somebody please think of the children?", which comes up a lot because it works, it really does shut down rational thought if you don't see it coming and block it.)
Same reason they spam this in email, and only some of the spam is "correctly" labeled.