Lips That Grip is one of those subreddits that has developed a strong cult following simply because an idea can exist and be shared regularly by those that are fascinated with its occurrence. Basically, the point of the subreddit is to share images and videos of girls with tight pussies that get fucked and at some point during the dick being pulled out, the lips of the pussy grab on as if to say 'you're not going anywhere'. It's much more common than you would think, and these guys absolutely love the fetish to death.
The current subscriber count for /r/LipsThatGrip is 71,000 and new content is submitted on a daily basis. Both amateur and professional shoots are encouraged as well as GIFs, videos and images that display that pussy-gripping goodness. Believe me when I say that even if you don't like the sound of this niche at the moment, it won't take too long for you to get a great level of satisfaction from the concept of a tight pussy gripping onto a rock hard cock.