**On the specific topic of gay “marriage” in EVERY country.
I think perhaps the best argument here is simply this.**
It violates the sovereignty of a country to impose a marriage law, and the UN cannot impose such a law on any particular country. Each country under international law is sovereign and according to its sovereign rights no foreign force can impose a law. Thus making the whole proposition that EVERY country can be made to impose this preposterous law simply that, preposterous, borderign on illegal. The best they could legally come up with is a voluntary resolution on the topic.
This also brings up a secondary argument that not all countries hold to western secular sociological values. For example islamic countries and many countries in the developing world hold to a mucch more conservative moral standard than much of the west. Likewise they are culturally different. Combining this with the issue of national sovereigty seeking to impose gay marriage universally despite the cultural and societal differences throughout the world in addition to violating the principle of sovereignty is cultural imperialism and an imposition on anothers society by a foreign force to further enforce a colonialistic western mindset.
To finish up only to conclude should you add a religious element to the debate by mentioning that many countries, societies such as those of islamic states and many christian states would be averse to accepting such a law, making such a forced imposition not only legally shoddy due to violations of sovereignty, but also culturally imperialistic and unenforcable.
Hope this helps smash that particular godless agenda