Colorado is one of the few states that still recognizes common-law marriages. "A common-law marriage is established by the mutual consent or agreement of the parties to be husband and wife, followed by a mutual and open assumption of a marital relationship." People v. Lucero, 747 P.2d 660, 663 (Colo. 1987). If you cohabitate (for any duration) and you both mutually and consistently hold yourselves out as "married" and have a reputation as being married, then you have a common law marriage.
The "mutual" component means that you both have the intent to be married. The marriage is agreed upon and entered into by both of you; you may or may not have had a marriage proposal, engagement, or ceremony. Courts have found that a common law marriage requires steadfast, unvarying, and unwavering representation and introduction as husband and wife. So, your family, neighbors, co-workers, children's teachers, etc. believe you are married.
The "consistent" component means that you claim a marriage exists for all purposes, and not only limited or just some advantages, occasions, or appearances. The courts have held that a common law marriage is not proven where parties hold out as married on only certain occasions where it is economically advantageous or there is a financial convenience, such as obtaining health or life insurance, but not for others, such as when filing income taxes and sharing bank accounts. CAVEAT: In view of modern trend toward cohabitation without marriage, clear proof of intent to form a marriage is required.
The test for determining whether a common law marriage exists as articulated by Lucero applies to same sex relationships, but the test should be applied consistently with the realities and norms of a same-sex relationship pre-Obergefell v. Hodges 576 U.S. ___ (2015). In re the Marriage of Hogsett, 2018 COA 176. The Supreme Court holding of Obergefell applies retroactively to Colorado same-sex couples in deciding whether common law marriage exists between them.
If you have entered into a common law marriage, then you are required to exist through an official dissolution or marriage proceeding. This will protect your interests, property, is the only way to avoid a claim to a longer duration to the marriage, and is required before you get married to someone else in the future.