Can an employer *force* one to take dental insurance coverage?

Me and my wife currently have insurance through my employer. She just joined a new job and her employer is offering health insurance: medical + dental. Our current plan has much better coverage for both medical and dental as compared to her new employer’s plans. So we want to keep the current setup and not sign up for the ones offered by her employer.

Her employer is allowing her to waive off medical, but not dental. She talked to the HR, and they said she must enroll, and if she doesn’t do anything, then she will get automatically enrolled in the default dental plan.

Does this sound reasonable? How can they force us to join their insurance plan?

The dental plan they offer is free, so one may think why not get the free stuff? If we take her employer dental plan, it will become the primary plan, and my employer’s will become secondary. For us, it’s going to complicate coordinating with both plans. And in the end the combined amount paid by both plans is not going to be more than the amount being paid by my employer’s plan, so I don’t see a point of complicating the billing. Let me know if I am not thinking straight here 🙂