Just for clarity, marriage is generally a net benefit when it comes to finances and taxes. However this thread has identified a situation where that may not be true: two people that have similar salaries which bumps them into a higher bracket. The realignment in 2017 improved that situation for lower income couples, but it's still an issue for couples where both are in the 33%+ brackets. But even if you are married, you can avoid this by filing separately. The reason the vast majority of couples don't do this is because "married filing jointly" saves the couple money as compared to "married filing separately".
That being said, it is (as others pointed out) a poor reason to get married. But it's also a somewhat lazy argument to use against getting married.
The MFS brackets are different than the Single ones, and this is not true. The 37% tax bracket starts at $510k for a single person, but $612k for a married couple (not double like all the other brackets) and $311k for married filing separately. This is only an issue if the couple (together) make >$612k.
In addition (this is not a comprehensive list):
1) The additional medicare tax starts at $250k for a joint couple rather than $200k for an individual - which means that potentially up to an extra $150k is taxed there.
2) The $10k SALT deduction cap is the same whether you're single or MFJ, going down to $5k for MFS
3) Either both partners itemize or neither does - no way for one person to take the standard deduction. Two single people could group all tax deductions under one partner and have the other take the standard deduction.
4) There's weird stuff with HSA caps, particularly if you have kids. The joint HSA cap is exactly double the single one - but if you're both single and have family HDHPs, you might be able to get a "family" deduction for one person and a "single" for the other (leading to 1.5x higher HSA cap) or even sometimes "family" deductions for both single people (leading to 2x higher HSA caps).
5) There's even weirder stuff with how IBR interacts with incomes - this is the one situation MFS typically beats MFJ, but single would beat both.
For a couple who make total >$250k/year, particularly if they earn similar incomes to each other (rather than one high earner and one low/no earner), there is zero tax advantage to being married - and in fact there can be a significant tax penalty. Particularly if they have kids - if they were divorced on paper, one could file HOH with the kids, the other could file single, AND they could group all the itemizable stuff under one person and have the other take the standard deduction (or just both itemize - now with both being able to count $10k in state tax individually).
The people who get tax benefits from marriage are those who have disparate incomes - then the benefits are huge. For people who earn similar amounts, it's typically a wash - unless they're high earners.
It's not a big enough difference for most given the other gazillion benefits of marriage... but it's certainly a consideration.