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.

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

Edit: Removed paywalled link about history of marriage.