You usually have to complete events/quests to unlock a character for marriage. For example, the wiki says this about Vampire’s marriage:
Vampire's wedding on the marriage screen becomes available after Mira's wedding is done. It requires the previous events of Vampire to be done, including the encounter with Neyain Kagabangui, the dragonic event which unlock Ryen's dragonic powers, and the blowjob with Mira and Vampire.
The wiki should have all events and requirements listed.