How does one sex a file? (A little carpentry humor.) ;-) The term "bastard" -- or "whoreson" -- is clearly defined in English Common Law. It refers to any person born of parents who are not married to one another -- any child born out of wedlock. Those who bore a child out of wedlock, but then married attempted to avoid the derogatory life-long label. However, there was no lawful possibility of the legitimization of a bastard after the fact. The harsh reality is that the child remains a bastard. The sex of the child is completely immaterial, although it is often enforced on males more frequently than females.

The term "bastard" might have been derived from an old french phrase "fils de bast" which means "son of the saddle," indicating that perhaps the biological father was a traveler, or that, according to the Online Etymological Dictionary, saddles doubled as beds therefore suggesting that one was conceived on an "improvised bed" in the heat of passion, something that would obviously not be necessary to married persons. Either way, that word "fils" in there implies a male son, as it is kind of like how we Americans use the word jr to indicate the heir to a father's name and property. Women would not get such a title, having not been the regular recipients of legal leavings. The word bast, meaning saddle, apparently had the pejorative ending -art added to it to give it particular poor taste (think something like saddle-tramp) and thus "bastard" was born, already carrying a meaning that had no application to females. Any usage that carried forward would have come from the original, and therefore still adhere to similar usage rules, hence only aplying to men, although this may change with time. But you may as well be asking why you don't call women "dicks."

Because language evolves and a person who behaves in an unkind manner would be described as a bastard if they were male. The female version would be bitch - which obviously means female dog when taken literally. It doesn't have to be logical - that's the beauty of language.