No. It is not discrimination. It is up to the individual insurance company to determine who they will cover - they are not discriminating based on gender, age, sex, ethnicity, etc.
I am not sure there is enough information to give a really good answer. But a company that is "self insured" still has a policy that provides coverages and still has employment policies. If either the company employent policy offers insurance to the spouse or the insurance policies offer insurance to the spouse, then they must offer it to your spouse. Marriage is marriage regardless of whether you are common law married or have a marriage license. I think this is an employment issue and if the company denies you a "benefit" that any other married employee receives, then they are not paying you wages and benefits to which you are entitled and are potentially violating the Colorado Wage Act. This is wage and benefit issue and the company cannot refuse to pay a wage or benefit available to all spouses simply because it does like the way you were married to your spouse.
Answering this quetion does not establish an attorney client relation. The answer is for educational purposes only. You should consult an attorney for your circumstances.
I think the company has a problem understanding what common-law marriage is. Common-law marriage is a way of getting married. It's not a "type" of marriage. A marriage entered into via common-law in Colorado is valid in every state thereafter until death or divorce. There does not seem to be any legitimate reason to differentiate based upon how you were married.
Of course, you could always simply go to the county clerk and get a marriage license. You can sign it yourself in Colorado (you don't even need an officiator) which would solve the problem. However, it sounds like a pretty stupid distinction. Frankly, if the policy says that they cover married people, I can't see how they can discriminate based upon how you were married.
You can reach Harkess & Salter LLC at (303) 531-5380 or info@Harkess-Salter.com. Stephen Harkess is an attorney licensed in the state and federal courts of Colorado. This answer is for general information only and does not create an attorney client relationship between Stephen Harkess or Harkess & Salter LLC and any person. You should schedule a consultation with an attorney to discuss the specifics of your legal issues.