When providing villagers with enough food, beds and workstations (not sure if all of them are required), they show the breeding hearts, but the process never finishes: Instead, they stand still for minutes until they get distracted by work tasks or the setting sun.

To reproduce, just fence in two villagers with a house with a few beds nearby, build some workstations and throw bread at them. Usually they start within a few moments if you execute /time set day.