Question: Will my group health insurance cover my partner, even though we're not married?
Answer :

In many cases, no. However, check with your employer. A growing number of employers are offering domestic partner benefits, including health insurance, to the unmarried partners of employees.

If your employer does offer health benefits to your partner, but your partner is covered by other health insurance, you'll need to decide whether it's better to be covered under one plan or two. A major disadvantage of a domestic partner plan is that the health coverage your employer provides to you and your partner is taxable income to you. You'll need to compare both the cost of each plan and the coverages they offer, taking into account the additional income tax you'll pay if you opt to be covered by only one plan.

You also need to realize that you will have to identify your partner and possibly sign a statement about the length of your relationship. Consider whether you are comfortable revealing that information.