To tell the truth, I think that the topic "Is Hindu marriage moral" is getting at the wrong thing.
Faith doesn't thinks gay marriage is immoral because she thinks homosexuality is immoral. This is because the Bible declares homosexuality to be immoral.
Hindus having sex isn't called immoral in the Bible, and it isn't immoral in Faith's book. Being Hindu (Well, being of a false religion) is immoral in Faith's book though ( I think) .
So the issue isn't "If homosexual marriage is immoral because of the Bible, so is Hindu marriage". I think it's more correct to say "If homosexual marriage is immoral because of the Bible, so is Hinduism".
Of course, this still gives problems to Faith - why does she want homosexual marriage outlawed because it's against her religion, but not Hinduism outlawed because it's against her religion?
Hope that made sense!