As a huge South Park fan the only thing you are waiting for is Eric learning his lesson. As time goes by the viewer more and more abandons that faith. "Cartman sucks" doesn't 'hit' it, but it comes close to satisfying the urges of a fan seeing Cartman being humiliated by his own stupidity.
Frankly, I liked it! The more you know Trey and Matt (creators) the more you know about their multiple alter egos being represented in this show. Eric Cartman obviously remains a focus, and in this one he's getting it, but somehow out of his own fault.
Taking a picture of him having his penis stuck in Butter's mouth while holding his thumbs up may appear perverted in THIS context... But if you try to get into the minds of children, it may sounds reasonable. It works, as politically correct I'm trying to be in my awareness, this episode is not immoral or sick, no, it just shows a classical situation of misunderstanding that most of us have probably already been into. Definitely in a similar context but maybe in a similar feeling.
I liked it... All the time while I was confronted with these scenes I thought "Why not?"... He's just a boy (at least that's what we're supposed to know)!!!!