Same sex relationships
While heterosexual couples have the option of common law marriage, ceremonial marriage and even no marriage, marriages between gay or lesbian couples are not permitted in Colorado. Colorado law requires a marriage to be between one man and one woman. C.R.S. 14-2-104(1)(b).

Moreover, Colorado will not recognize as valid same-sex marriages performed outside of Colorado, even if the marriage were legal under the jurisdiction where it was performed. C.R.S. 14-2-104(2). So a same-sex couple who married out-of-state is unable to obtain a dissolution of marriage in Colorado.

Civil Unions
The Colorado legislators have considered a law to allow non-married partners of either gender to enter into civil unions that would provide the rights and responsibilities of marriage to the participants.  The statute was killed in committee in 2011 and will be considered again in 2012.  Presently, people who are not married under the statutory scheme of marriage have to write their own agreement to define their rights and responsibilities. 

Same Sex Marriage Alternatives
Designated Beneficiary Agreements. It's not a domestic partnership or civil union, but it does give unmarried couples, including gay & lesbian couples, the right to receive death benefits, inherit without a will, make medical decisions, etc.

Allocation of Parental Responsibilities. Same-sex and other un-married couples can still share parental responsibilities, and have court-ordered parenting time if they break up.

Same-sex and other unmarried adoption. Colorado law allows any two adults to adopt a child.

Agreements can be created for married, unmarried and same sex couples that define their legal rights and responsibilities.  These agreements can be written before or during the relationship between the parties.  These agreements are binding on the parties as a contract and can be made into an order of the court in a dissolution or partition court action. 

Collaborative Process/mediation can be helpful in creating these agreements because the parties will consider all of the issues that are raised by combining their household and jointly taking on the purchase of homes or raising children.  The parties will consider their values and plans to create a document that meets their specific needs. 

More Information
Wikipedia has full discussion on the Defense of Marriage Act, including its legal challenges.