Acceptable use is a figure of speech. I did not search extensivly for any definition that itch put forth, what must be marked as adult.
But as long as your game is not intended as a porn game or an explicit horror game, it most likely is non adult. Just imagine what rating it would have, if it would have an official rating. If it would be 16 or less, chances are, it is "non adult".
Of course, cultural values are different everywhere. In Europe, naked boobs would get you to maybe 16, depending on context, and violence will quickly get you to 18. In US it is the other way round. Violence seems ok for kids, but beware the occasional naked pixel bosom.