Must an employer's group health plan recognize an employee's in-state marriage?

My wife and I have been married since 2012. We lived in Washington, D.C.

We established a comomon-law marriage, which is recognized under D.C. law. The web site of the D.C. Department of Health confirms this is true.

We refer to each other as husband and wife, we live together, we have a joint bank account, I'm in her will and she's in mine, we own a house together, and we file taxes as married.

My wife's employer's group health insurance plan does not recognize our marriage and will not let me enroll in the plan under spousal coverage. We have already provided a notarized letter stating that we are married, and we also gave them a copy of our most recently prepared tax returns showing us as filing as married.

Her employer is a private corporation. It is not religious or connected to any church in any way, shape, or form.

The plan is located in D.C. If it was an out-of-state marriage, I might say they can refuse. But it's an in-state opposite-sex marriage. The plan is refusing to recognize our in-state marriage.

I can only imagine that ERISA applies here, and that ERISA requires a plan to recognize marriage law based on the law of the site of the plan, which clearly recognizes our marriage.

I could use some help please? Relevant laws, court rulings, and so forth requested in particular.

Thanks much.