New Year's drama with my girlfriend's parents?

My girlfriend and I went to separate New Year's Eve parties but I left before midnight so I could kiss her and then come back. Earlier that night she had asked her mom if it was okay that I drop by, but then five minutes before midnight her mom finally answered, saying that it wasn't okay and that she was coming to pick her up. My girlfriend got really upset, and as a result I'm pretty torn up about it too.

We're in high school (Catholic, of course), but I'm two years above her. Her parents aren't really happy about the age difference, or the fact that we're both girls, but they've been accommodating and seem like nice people.

Now her mom says that she's betrayed her trust, and it doesn't help that her parents are slightly homophobic. They know we're dating, and I want to make things right with them, but I don't know how. Any advice?