in all seriousness though i don't know why americans find nudity so offensive (i am american before you get mad). it's literally the most natural thing a human can be.
As a genuine old fart, let me answer that question:
It's an old social "norm" designed to protect all of us.
I'd wager that most of you wouldn't want to see 75%+ of the rest of people nude. Open up that door too wide, and you'll likely end up blind (however "natural" what's behind it might be).
Secondly, not knowing what's inside a wrapped gift is fun, and unwrapping the gift is exciting. If everyone just hands you your present, eventually you'll be less excited about the next one.
That answers the above question, but doesn't really explain why we can't have a bit a titillation in a game designated "M".
Disclaimer: When I was 19, I'd have never believed any of this...