Because your health budget is a CHOICE plan, IRS rules require that a domestic partner be a tax qualifying relative for you to use your health budget on their costs. This means you must provide more than 50% of their financial support during the tax year.
This is different from a legal spouse, who can be covered without meeting the qualifying relative test. If your domestic partner doesn't meet this requirement, they can still get coverage, but you'd need to pay for it with post-tax personal funds.