Do you live in California? Or another state that allows domestic partners to file a joint return as if they were married?
You are unmarried for federal tax purposes. I believe the IRS used to make allowances for domestic partners, but since the Obergfel decision, and anyone can get married, there are no longer shared federal tax benefits for people who live together but don’t get married.
You would file separate returns as single, unless you provide care in your home for a qualifying child dependent, in which case one of you could file as head of household. You may each list your own medical expenses as itemized deductions. You can only deduct medical expenses you pay for yourself, your spouse, or someone who is your tax dependent. Your partner can’t be your tax dependent if they have more than $4300 of taxable income.
Regarding loans, interest on most loans is not tax deductible, only on a home mortgage. If you pay interest on a mortgage, you can only deduct the mortgage interest if you are listed on the loan as a borrower and are obligated to pay interest. If you are both obligated to pay interest, then you each deduct the amount you actually paid. You can probably get away with listing all the mortgage interest in the name of one borrower, because the IRS will probably allow you to treat the interest as if it was a gift from one person to the other and then the other paid the entire expense.
Regarding fertility treatments. This is controversial but the current federal law is that you can only deduct medical expenses that you pay to cure, treat, or mitigate a disease or condition of the body. The IRS has interpreted this to mean that if you are healthy and you are paying for services such as artificial insemination to have a child with a same-sex partner, this is not a deductible medical expense. Surrogacy expenses for someone else are also never deductible medical expenses, because you can only deduct medical expenses for yourself or your spouse or your dependent. If you are paying expenses for a surrogate to have your child, those expenses are never deductible even if you require surrogacy because of your own health conditions, because the surrogate is not your spouse or your dependent. This is how the IRS and the Supreme Court have interpreted the current tax law; if a change is needed, it must be made by Congress.
In some states, you may be allowed to file a joint state tax return even though you must file separate federal returns. We would need more information about where you live in order to determine if this is appropriate for you and to help you do it in TurboTax.
