Marriage in Western Liberal Democracy

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Marriage in Liberal Democracy

Same-sex marriage is legitimate for liberal democrats on the basis of freedom of sexual orientation.

I wonder what will happen when some people among the seven billion inhabitants on Earth come anywhere with an argument that it is their nature to have family union with multiple mixed or same-sex group of spouses.

Shall marriage be legally and socially acceptable between more than two persons?

What is the maximum number of partners in a single marriage?

And, should legitimate marriage have any gender and/or size specifications?

Can liberal democrats come with an acceptably convincing and practical definition of marriage in their respective countries or globally?