I noticed this a few times. The user would ask a question, and only upvote their gender, and not upvote the other. Even when nothing is said really, they'll upvote them and not someone from the opposite gender, who gave a thoughtful response.
I haven't done that. If I upvote someone, I liked their answer due to it's poignancy or I thought it was funny. If I ask a question, I usually give everyone an upvote that actually gives a coherent response as a 'thank you,' I guess.
I did use to downvote only girls though. Cooties and such. I don't approve.