Why did the cartoonist use marriage in the cartoon "Interrupting the Ceremony"?

From the US's inception, concerns about becoming mired in foreign commitments were a consistent issue among many Americans and their elected officials. Among the Founding Fathers, Thomas Jefferson, principal author of the Declaration of Independence and contributor to the US Constitution, repeatedly warned against "foreign entanglements" such as had repeatedly throughout European history drawn those mostly monarchical regimes into and out of endless wars. Jefferson's admonitions against alliances that could and likely would similarly draw America into European conflicts set the tone for US foreign policy for many decades to come. At the bottom of this answer are two quotes from Jefferson illustrating his concerns about the forging of relationships abroad that went beyond the merely commercial.

Moving ahead into the 20th century, the isolationist sentiments that dominated public discourse with respect to foreign policy remained strong. President Woodrow Wilson, in office at the time of the Great War's start, had hoped to keep the United States out of the war engulfing Europe and to craft a new international structure that would help prevent major conflagrations moving forward. That structure would become the League of Nations, an anemic precursor to the establishment of the United Nations that took shape out of the ashes of World War II.

While the isolationist sentiments that had characterized American foreign policy with regard to Europe held sway, German submarine activities and German Foreign Ministry intrigue (i.e., the "Zimmerman Telegram") drew the United States into the war anyway. Establishment of the League of Nations, however, would be President Wilson's crowning achievement. Or it would have been his crowning achievement if the Republican-controlled Senate had agreed. And this is where the political cartoon comes into play. The cartoon depicts the "foreign entanglements" against which Jefferson had warned us, personified by the bride, with Uncle Sam depicted as the groom. The clergyman, representing the League of Nations (hence, the volume he is holding rather than a Bible), is officiating at a union of the United States and the foreign entanglements that US policy had for so long eschewed. Just as the "clergyman" is uttering the traditional opening for anyone opposed to this union to "now speak," an individual representing the United States Senate, the chamber of the US Congress vested with the authority to ratify treaties with foreign nations, bursts into the room, disrupting the ceremony.

Despite its substantial role in creating the League of Nation, the Republicans in the Senate succeeded in blocking US accession to that organization, thereby dooming it. As the cartoon suggests, the US Senate put a stop to the marriage or union between the United States and the League of Nations.

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"I know, too, that it is a maxim with us, and I think it a wise one, not to entangle ourselves with the affairs of Europe. Still, I think we should know them." --Thomas Jefferson to Edward Carrington, 1787. ME 6:396

"Nothing is so important as that America shall separate herself from the systems of Europe, and establish one of her own. Our circumstances, our pursuits, our interests, are distinct. The principles of our policy should be so also. All entanglements with that quarter of the globe should be avoided if we mean that peace and justice shall be the polar stars of the American societies." --Thomas Jefferson to J. Correa de Serra, 1820. ME 15:285