"Teenage Mutant Ninja Turtle" is an insult here because the translation, "忍者神龟", if you break it down, "忍" might mean shinobi, but it also means "to endure", and "龟" might mean turtle, but it's also an euphemism for "cuck", so putting it together, you have someone who's, for some reason, knows that their partner cheated on them, but accepts it and won't do anything about it