That's a fair question, and one of the FAQ. I should really just make one at this point. You asked nicely so here's a fair answer. I post what I post because I enjoy posting it. That part is simple. The "porn" part is a hot mess. Imgflip is not a pornographic website. Great, so what's pornography? It doesn't say. The dictionary usually includes a sex act but at a minimum (as clarified by the imgflip gods, i.e. owners and global mods) involves exposed private parts (genitals, buttholes, developed female nipples, ejaculate and possibly pubic hair) with the intent to cause sexual arousal. Censoring these parts doesn't help because censored porn is still porn. (It's also worth noting that "porn" is only relevant when taken as shorthand for "pornography". The definition of porn that refers to sensationalized content "food porn, horror porn, etc." is very much not what TOS means when it says pornography.)
Butt cheeks do not meet the definitional criteria of pornography. There. Read it. Soak it in. It's a fact. It's what the rules are built around. Denial doesn't help. Getting angry changes nothing. The second component is the No Nudity rule, which says that bikini girls etc. are fine so long as all parts are completely covered. They don't specify what they're covered with, but camel toe is allowable so basically anything as long as it only shows an outline. My memes. Listen to me: ALL MY MEMES COMPLETELY COVER ALL PRIVATE PARTS. If they don't, flag them, report them, request they be removed because THEY SHOULD NOT BE HERE. Hear that? I DO NOT WANT PORNOGRAPHY on this site. Let me say that shit again: I WILL NOT POST PORNOGRAPHY ON IMGFLIP. The erotic content I do post goes against the spirit of the site, but so do insults and bullying, and that's like 3/4 of MSMG, so look in a go***mned mirror if you're so concerned about following rules to the letter.
Tl:dr, the content I post isn't pornography go***amnit, and I post it because I want to. Report me if you think it'll help and then stick a tampon in it and move on.
And get some perspective while you're at it. Cartoons I post get 10x more negative attention than real life images and they're drawings. D**DOS are allowed on imgflip for christsakes, and ya'll are pissing yourselves over a cartoon butt or anime chick in a bikini. My question to you is, why is this such a big deal to you?