My girlfriends Christian parents disowned her after she came out and their relationship is ruined. Why do I feel like this is my fault?

Unfortunately, many LGBT people feel guilty for things they should not be blamed for. You're not alone in this! There are, sadly, many families who shunned their LGBT kids. The only thing to blame is their own intolerance, and their inability to love their kids more than they love their own prejudice. Your girlfriend's orientation would be the same even if she hadn't met you, and she would have had to face all of this anyway, sooner or later. All you did is giving her the love her own family couldn't give her. Hold on to each other. Be her strength through all of this. It's a hard moment, but with you by her side, she can go through it!