Why does "bad guy" have negative connotations, but "bad girl" tends to have notes of sexiness behind it?

Something I never understood, but yeah, when an adult is labeled as a "bad guy", they are considered a villain or a negative influence, but call a grown woman a "bad girl", and it sounds flirty and naughty in a sexy way. Of course, this doesn't hold true when talking about a child misbehaving.

So why is it that way? Your thoughts?
Why does "bad guy" have negative connotations, but "bad girl" tends to have notes of sexiness behind it?
Post Opinion