Captcha are these words you have to type on some webpages in order to prove you’re a real person and not a bot. Usually made of two words picked at random, captcha sometimes make a funny sentence. And when that sentence is used as a punchline in comic strip form, you get a captcha comic. Like these;
The funny part about that last example: I just typed Stan Smith Roger on Google Image, without any filter on, and that was one of the first pics appearing. I was just looking for a smug looking Roger with a finger pointed up, but hey, if Hentai suits my needs better, who am I to turn it down?