Catholics answers Plz.?

Well, I was asked to be a witness for a marriage. My best friends little sister is getting married to her GIRLFRIEND. I have no problem with her being gay. Its her choice. But would it be going against my church to be a witness to a marriage? My name will be on the certificate. I'm torn right now. I love my friend. I love my church. I have to be at the courthouse in 3 hours. I don't know if I should go or not.