You ever just get a gut feeling? One that you know isn't fair or right, but it still arouses emotions all the same? Something like seeing your girlfriend's ex and knowing he was inside her once, even a long time ago, and it fills you with a twinge (or larger) of judgement/disgust for her or even an anger for him?

Now imagine if you didn't ever confront or dismiss those, you instead acted on them every time. If every source in the world told you to acknowledge and champion them.

That's how these double standards are born. They act first on instinct, then back rationalize it when questioned.