I know this one! Not all countries provide tax deductions for marriage. I was going to say there is not one in Japan, but on looking it up, it is more complicated than I care to look at. (I understand spouses tend to have separate finances, but I am far from Japan.)

The historical lens on taxes and marriage is also as complex as one might think. random ref I didn’t read

Point being, in your setting, marriage does not need to have any tax implications at all.