I'm sure the reasons you listed above were used when proposing new taxes, but I'd wager your motive is wrong. I would bet (without having any evidence), that the motive was simply to generate more tax revenue. Like most taxes, it's hard to get through if a majority of the voting base is affected, so it was probably easier to raise taxes by excluding people who are married (the majority), so that the government could collect more taxes from those who aren't (the minority). I could do a quick google search to see if I'm right, but I felt like rolling the dice.

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