You're allowed to show it so long as there's:
1. No Nipples
2. No Bobs-yer-uncle or Wee-willy-woo-woo's (You know what I'm talking about.)
3. No blatant sexual activities, unless outlandishly hilarious and done in moderate taste (see the animated Loce Hina gif in the Not What They're Really Saying section, around page 2 or 3). This means no hentai.
I'm sure you can get away with the buttcheeks, but you might want to check on the whole "total nudity facing back," though that's probably okay.
PLEEEEEASE let it be okay

You know what they say: "when life gives you a T-Rex, go ninja-kick it in the head." - Rayne Summers, Least I Could Do
Proud to be a Jenova's Witness - WWSD (What Would Sephiroth Do?)