Nobody has ever said that every sexual orientation (no preference) should be a component of marriage. Plenty of countries have legalised gay marriage without legalising zoophilia, paedophilia, or whatever other slippery slope you want to bring up.
How does gay marriage devalue marriage? Bc gay relationships are inferior in your eyes? How?
Zoophilia is a kink, there is no such thing as a natural orientation exclusively or even partially to animals.
If marriage is by default religious to you, what about marriage between atheists? Are they devaluing marriage?
And do you really see gay relationships as on a par with beastiality?
No church is forced to perform gay marriage just bc civil marriage is legal for gay couples. Civil marriage is none of the church's business, we don't live in a theocracy.
Plus some churches, eg. Quakers, wanted to perform same sex marriage.