Well animation is just a medium.
Say you make an animated film in the vein of “Fatal Attraction” or “Disclosure”. Those types of stories can use sexual content in effective ways.
You also have things like “Dirty Dancing” that kind of cross-over with romantic physical intimacy.
And if you turn the dial down a bit further you have things like Underworld, Resident Evil “Alice” movies, or Wonder-Woman where there is this inter-breeding of muted sexuality and action themes.
Which to me… all the above are fair game.
I think an animated film about a Bank Robbery can be a good one too - cause these are situations where you might have something to say.
Another classic example of “something to say” is Brian De Palma’s “Scarface” which was bordering between an R and an X rating. Supposedly law enforcement groups in Miami said the film helped people to understand the bad effects of drugs because of its graphic portrayal of the multi-layered downward spiral of the lead character so the ratings board were persuaded to let the film have an R rating and go to theatres, where an X rating would have reduced its viewership.
It’s in the hands of the artists (especially in animation) to choose what they can do for “mature audiences”. The concept of mature audiences is that you are going to talk about things “Mature people can grasp”.
I’ve always thought that was the idea.
P.S.: “Invisible To You”, posted by Rob above, is a brilliant series, but if I may be frank… I’ll be damned if these are the only forms of “mature animation” people will allow.