The reason there's a tax penalty for being married is because the income taxes apply to a couple and not just an individual.
Under a progressive income tax, a couple's income can be taxed more or less than that of two single individuals.
The marriage income tax penalty is 12% if you have no children and the marriage income tax penalty for couples with children is 4%.
You only have to file income tax returns and pay income tax if you make more than $1,050.00
If you earn less than $1,050.00 per month then you do not need to file an income tax return and you don't need to report your income and pay any income tax.
Even if you do earn more than $1,050.00 you may or may not owe any income tax but if you earn more than $1,050.00 per month then you still legally have to report that income to the IRS.
But just because you file an income tax return and report your income to the IRS it does not always mean you have to pay any income tax at that time.
According to the IRS you must file an income tax return if, you had more than $1,050.00 of unearned income (typically from investments).
Or you had more than $12,000 of earned income (typically from a job or self-employment activity).
Or your gross income was more than the larger of $1,050.00 or earned income up to $11,650.00 plus $350.00