The employer is the one who pays the bill. The insurance carrier never sees what portion is paid by the employer and which by the employee. All the insurance company knows is that they get a single check from the employer paying the bill. It's not broken down to show the employee's payment.
It has always been my understanding, but I have no proof of this, that if you are paying a portion of the insurance it must be optional. I cannot show you a law that says this.
How large is your employer? If very small, it's possible that their contract with the insurance company guarantees a 100% enrollment. If that is the case, it MAY be that with proof of other coverage you can opt out. You would have to discuss that with HR.

