The following civil unions are prohibited in the State of Colorado:
- A civil union where one or both partners are minors (under the age of 18).
- A civil union entered into prior to the termination of a marriage or civil union of either of the parties, unless the earlier marriage or civil union is currently valid between the two applicant parties.
- A civil union between an ancestor and descendant, brother and sister, uncle and niece, or aunt and nephew, whether the relationship is by half or whole blood.
Civil union between first cousins, whether the relationship is by half or whole blood, is permitted in Colorado.