What is funny? Jokes are weird to think about, but it generally relies on setting up an expectation and then surprising us in some way. Here it’s that when he uses the phrase ‘a perfect ten’, we assume he’s referring to a highly attractive (adult) woman, then in the next panel we see he means ten as an age, which gives our brains a little stumble, a mismatch between the pattern we were expecting/predicting and what happend. For some reason, this little thing of setting up am expectation then subverting it tickles our brain in a way that makes it a joke. Having the reveal also be a topic like child rape that is so taboo and so unacceptable just increases this effect of how unexpected it is, this is generally what ‘dark humour’ is going for (works for some people, for others it just takes it too far, to where their emotions/associations about the bad thing far outweigh any humour, and put them into a state where they’re not really able to find anything funny). Anyway, you don’t have to like it, but it seems pointless to try to argue that certain subjects are not suitable for jokes. Some people like these jokes, you don’t, and that by itself doesn’t make either of you bad people.