Is there a significant other law in WI for health insurance coverage?

There is a domestic partner law for same-sex partnerships. There is no law for opposite-gender relationships, i.e. Common Law Marriage.

However, state law does not apply to employer funded health care if the employer's plan is a) not written on WI paper (i.e. your company has a large office or corporate officer in another state and they bought the plan from that state) or b) self-funded (the employer pays the claims and uses the health insurance carrier for administrative services only).