This comes up a lot in Fallout 3, seeing as approximately half the NPC population are female.
I have to admit I feel somewhat more empathy for females whenever they suffer (whether they're my enemy or not), with the possible exception of raiders who are sadistic psychos and deserve whatever they get.
I don't shy away from combat, if I have to fight I go all out, but my reaction to having to kill someone because they attacked me unprovoked is usually something like "Why did you make me do that?!" or "Such a shame...", and there's usually an extra tinge of regret, however slight, if the combatant was a woman.
For instance the other day I was wandering around an old mansion on a windswept coastline and I was attacked by a female mercenary for no apparent reason. I defended myself and killed her, and immediately found myself wishing it hadn't been necessary, and as I stripped her of her leather armor so I could use it to repair mine, the indignity of it struck me, also. Such a shame...
And then I found her companion (also female) a mere 50 meters away, sleeping alone and totally unaware of my presence. She too was hostile and as far as the game was concerned I would have been entirely justified in slitting her throat as she slept, but I couldn't do that, it'd be an evil and unprovoked attack.