My wife and I have always used an accountant for our taxes, and each year they would look at both scenarios (filing jointly or filing separately). It always made more sense to file jointly. Granted, she never earned anything close to what I do, and now she's retired, so maybe it would be different in a two-high-earner household.
People commonly do not understand that married filing separately is different from filing single. The tax brackets are different, the eligibility for deductions and credits are different, etc. Married filing separately is much worse than filing as married filing jointly or as two singles in the vast majority of cases for two individuals making $100k or more a year. However, you cannot legally file as single if you are married.
Before the Trump era tax code changes, tax brackets jumped up for married couples when both partners made more than about $50-$60k/year. So if you had a housewife or househusband who stayed at home and made nothing or limited amounts, there was a benefit to being married. Alternatively, when two people making $250k came together (top bracket 33% as singles), they would end up in the $500k married bracket, which was the top bracket of 39.6% married. This ended up costing well over $10,000 a year in excess tax based on that factor alone.
Now with the Trump era tax code changes, the married tax brackets are much more forgiving. When two people making $250k come together and get married for example, they start in the $250k 35% single bracket and both end up in the same top married bracket of 35%.
Ok so the marriage penalty is gone you say? Not so fast in our case.
The child tax credit cap is $400k for married filers, $200k for single filers. That's nice, except my partner makes under $200k and our combined income is over $400k. Filing as singles nets us the whole $2000 tax credit there per year per child. Getting married would cost us $2,000 per year per child.
The standard deduction is now up to $12,000. It's $24,000 for married couples. Ok, so our house is in one of our names entirely. That means one of us as single can take the mortgage and property tax deductions plus other itemized deductions (~$40,000) and the other single can take the standard deduction ($12,000). By staying single this is a net deduction gain of $12,000 or almost $4,000 in the pocket per year.
So by staying single, we continue to legally avoid paying Uncle Sam about $6,000/year. This amount would increase if we had more children. Before the Trump era tax changes, we saved about $16,000/year by staying single and not being married.
I know... Everyone thinks I've misunderstood this. Others think our government can't possibly be penalizing people to be married. The reality is: "They don't think it be like it is, but it do". In other words, it's screwed up but it's true. I see no good reason why two working physicians should get married. There are no tax breaks and there could be significant penalties.
Edit: White coat investor recently had an article on this. He points out some other things I missed like my SO can claim head of household and get an even bigger standard deduction.
