once you get married with any NPC, the rest of NPCs you dated (either boyfriend or girlfriend) go back the state of single
but the bouquet marks still display on their profile photos
this is the evidence that you guys fell in love
that means you can marry any NPC and keep giving bouquet to those who is single
but he or she won't be your lover
nevertheless, you can still unlock the limitation of 8 hearts for single NPCs
don't be stressful, just keep cultivating relationships with single NPCs, there will be no moral issues