Interesting, Hydra. I ran "girl born out of wedlock" through Onelook's Reverse Dictionary; bastard was the first entry, and illegitimate was the second. I didn't see any def. of bastard that mentions gender, so it does seem kind of odd that it has come to refer to males; but I think Helen's post about inheriting could well be the reason.

Here's what the Online Etymology Dictionary has to say:
bastard
1223, "illegitimate child," from O.Fr., "child of a nobleman by a woman other than his wife," probably from fils de bast "packsaddle son," meaning a child conceived on an improvised bed (saddles often doubled as beds while traveling), with pejorative ending -art. Alternate possibly is that the word is from P.Gmc. *banstiz "barn," equally suggestive of low origin. Not always regarded as a stigma; the Conqueror is referred to in state documents as "William the Bastard." Figurative sense is from 1552; use as a vulgar term of abuse for a man is attested from 1830. Bastardize "debase" is from 1587.
Online Etymology Dictionary