I would like to point out that you care more about how you look to your friends then how uncomfortable this situation is making your girlfriend.

Your comments seem like you're trying to diminish your involvement because you're not posting or commenting on it however you're not saying anything about it being wrong, uncomfortable, or unwanted. I think in her mind that's just as bad as commenting or posting. Not to use a bad analogy but think about bully bystanders. Being someone who doesn't stop bullying when they see it is almost as bad as being a bully. You're not actively participating but you're also allowing it to continue by not saying anything or doing anything about it.

Again I couldn't think of a better analogy but just something to chew on there.