PC-PC marriage

In a long campaign, 2 PC want to get married in-game. DM refuses the request and demand they either drop the proposal/marriage narrative or quit. DM's reason is to forbid any mini-group within a group of players.

DMs on this subreddit, what would be your decision?

For context, i am neither the DM nor the player. But if I am the DM, probably I will allow the marriage..