invest into intrigue lifestyle

have your spymaster fabricate a strong hook on their father, and use the hook to make him agree to the marriage

i guess you could maybe also seduce your future spouse a bit too if you want so they dont hate you (optional)

if youre still having trouble - if youre lowborn, you can agree that all heirs will be of their house and it will cost you nothing except your children wont have the cool red-star-and-crossed-kalashnikovs coat of arms you made in ms paint