Many people today do not want to get married. They have long-term relationships, and they want to keep it like that. Marriage is not the only way you can make your partnership legal in Colorado. You can choose to register a civil union or domestic partnership at the local county clerk. All three share some rights, but there are significant differences. A family lawyer in Colorado Springs, CO can go into more detail, but here are the broad strokes.
The traditional way to make a partnership legal is to get married. You can get legally married in any state, and it will be legal in any state or country in the world. You can petition a non-citizen spouse. You can file for divorce in other states. You have extensive rights and duties at the federal level. Tax breaks and medical insurance are just some of them.
You can register for a civil union anywhere in Colorado. The state will recognize it and give you the many rights given to married couples. These include the power to dissolve the union in that state in a process much like a divorce. However, you have no rights at the federal level. Civil unions also have no legal standing outside the state. Not all states have a civil union registry. The only other states that recognize a civil union are Hawaii, New Jersey, Vermont, and Illinois.
A domestic partnership shares many things with a civil union. You have to register it with your local county. It only has legal status in that state. However, domestic partnerships are available in more states than civil unions.
Marriage has many benefits not given to people in civil unions or domestic partnerships. If you want to get some of these benefits, consult a family lawyer.