You are correct. If you meet the definition of a covered employee, they HAVE to offer you the opportunity to be covered regardless of whether you have access to other insurance.
I think your employers are a bit confused. They CAN have a rule that they will not cover a dependent, if the dependent has access to other coverage. So they can tell you that they will not cover your husband, since he has coverage available to him elsewhere. This is not only legal, it is becoming increasingly common.
But YOU are their employee, not a dependent of an employee. So failure to allow you the opportunity to join their plan would be a violation of ERISA, as long as you meet the eligibility qualifications.
Please do not ask me for a link or something you can print out to show them. ERISA is an enormously long and complicated law, and it would be nearly impossible to provide that.
But I assure you that if you called the DOL and reported to them that you were being declined the opportunity to join your employer's plan because they wanted you to be covered on your husband's plan instead, the DOL would support you and not your employer.
What I would do would be to go to your HR person, and, RESPECTFULLY, inform them that under ERISA you have the right to be offered the insurance benefit regardless of whether you have access through your husband or not, and you would like the information about the plan and the enrollment forms now, please.