I've heard a lot of people say this, and yet I say that they're wrong. Cartoons have always had sex in them. Just take a look at all those clips from Animaniacs. There's tons of innuendo in that show,and same with other shows from that era seem to have a lot of sex in them too. I think that its just we're old enough to see the hidden jokes that are there.
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I remember watching the third episode of batman beyond (recently) and catching some very blatant innuendo. It literally does not try to hide the fact it is innuendo.