Some time ago I posted about boy band and girl group being more common than boy group and girl band, and pondered whether it was simply added alliterative appeal, with no firm conclusions. Recently, for no apparent reason, I wondered whether bad boy(s) and good girl(s) follow the same pattern. Yes and no. Google Ngrams shows that bad boys and good girls are more common (in terms of usage, at least) than good boys and bad girls. In the singular, though, good wins, with good girl and good boy being more common than bad boy and bad girl. I can’t draw any conclusions from that. The 1995 movie Bad Boys is unlikely to have had a significant overall effect.
There is also guy, which complicates the picture. Good guy is more common than bad guy, but bad guys are more common than good guys. It would seem that girls are more often good, singularly or plurally, and a boy is more often good but boys bad .
There is/are also also Guys and Dolls, but good doll(s) and bad doll(s) are very rare.
Never Pure and Rarely Simple 