I left the beard the way she liked it. Now, one conclusion you might draw from this is that I care more about my girlfriend's emotions than your girlfriend cares about yours. Too bad for you. Another conclusion might be that I don't care about my appearance as much as your girlfriend does. It doesn't matter in the least.
Justification has nothing to do with it.
I know that if my girlfriend cut her long, beautiful hair while I was out here in Washington I'd be mad as hell. No, of course not. But our emotions are never reasonable. I'm willing to go out on a limb and say that your girlfriend has had a couple of these non-rational outbursts during your relationship. One of the things that makes relationships so difficult is tending to the other person's unreasonable emotional needs.