Apparently the regulations are as simple as "Can you see their panties?" Bayonetta does all kinds of suggestive poses, skin tight clothes, bare back, violent gunplay, etc, but her panties are definitely not visible, thus she's okay. Even Palautena's outfit had to be carefully modified to pass.
@Bug Leader Yeah, I'd legitimately like to hear Cero's thought process on how Mai was unacceptable but Bayonetta was fine. Especially since she does both sexy AND guns, which I thought were 2 big No-nos on the list.
To be fair, the 'good boys and girls' line was all localization. In the Japanese interview, he basically threw shade at Cero, suggesting that it had been implied that including Mai would have gotten the game's rating bumped up. He seemed pretty annoyed about it, being an SNK fan and all.