Yep given your situation, I was just guesstimating how much you'd lose in your refund if you were married. That is what seems wrong to me, a tax code that is set up which in a way discourages marriage. Reason for that is the tax code is still living in the 50s, when Ward goes to work and June stays at home. Not all deductions are double for married taxpayers, items phase out sooner, and the ability for one to itemize and the other take the standard deduction (which is not allowed even for Married filing Separately).
As a CPA given your situation, what would concern me is taking the Head of Household when your partner is taking 100% of the mortgage interest, plus probably 100% of the real estate taxes. To have HoH status you must pay more than half of the expenses for keeping up the home. That is why only one of you may claim that status. I'm guessing you take the standard deduction, while he itemizes. I think these items are why some CPAs are commenting, because the scenario sounds a bit "aggressive" from a tax standpoint.
People will say they got rid of the "marriage penalty", but this is a prime example of it, one person being able to take all the itemized deductible items (except for the partner's state & local withholding), while the other takes a standard deduction. It used to be even further biased when the tax tables were lopsided. You would have people divorce the last week of December. Get married and file their taxes in January. Then pocket or take a nice trip on the tax savings.
Noticed you mentioned daycare expense. Do either of your workplaces have a dependent care plan? That was a nice little benefit for us. DW now can withhold up to $5,000 per year to pay for daycare. That $5,000 is not subject to income taxes, and also is not subject to Social Security taxes.