We can't expect everything to be roses when two become one, and taxes after marriage can be surprisingly complicated (not that taxes are ever a simple cup of tea). Combining two salaries means you have a bigger pot to brew, but it could also mean that you end up paying more taxes as a married couple than when you both were single. Or it could mean less taxes. See what I mean — complicated. Find out more about the marriage penalty when you read more.
The marriage penalty comes in to play when both spouses work and have to pay a higher joint tax rate on the same total income than they would pay if each were single. This penalty typically kicks in when both spouses have relatively equal incomes. So, married couples can end up paying more than they used to because their combined income pushes them into a new tax bracket.
Depending on your situation, some of the deductions that may apply when you get married (like sharing deductions for children and mortgage payments) could actually outweigh the penalty. In order to get a good idea of how getting married may affect your tax situation, play with the numbers yourself or ask a professional to help you wrap your head around all of it. Personally, I'm planning on asking a tax expert to walk me through it. I don't want to miss something and then be surprised with a larger tax bill than expected.