On a natural basis, it is an issue of national defense / national survival. There are clear and compelling religious and supernatural reasons, but for those to whom religion is an anethma in the public discourse, the demographic collapse ongoing in Russia and Japan and the EU (excepting muslims) ought to be a sufficient cause for alarm.
Yes, you are right that counteracting demographic depopulation would be another public policy reason to encourage civil traditional marriage beside providing the most secure environment for children.