you have to complete the priest's quests. Craft the Angel Statue (i think u get the crafting recipe by buying a book at the General Store) and then another priest quest to donate 20k. Then next time you talk to your "Lover", it gives u the Propose option.
But! before you can unlock the priest's quests, u need to finish the lovel hotel quest with the two detectives. That requires your Charm to be a certain level so you can convince the love hotel owner.
There's other pre-requisites like you need to have bought the "Big Bed" them eventually the "Engagement Ring" from Grizz