Ah there's the problem. If you marry someone you get locked out of the other relationships and won't meet those characters. I thought I had put that in the description or something but I will need to double check everything and update what needs updating after work.
The intention behind that mechanic is you explore, meet the characters, progress what relationships you want then pick one character to commit to for the end game content. Its not intended to be a harem game - technically, though I suppose it can be viewed as such in the mid level relationship tiers. Obviously haven't got that far with the main story/end game stuff yet but the characters and systems are built around that rather than trying to tack that system on after the fact. Sorry, probably a bit frustrating until there's more content.
Hopefully, you've got a save from before getting married - that will unlock all the other characters again. I'm working on a "cheats" system for a future build because I know the RNG exploration can be a PITA but need to iron out how it will work without breaking the game.