The following marriages are prohibited in the State of Colorado:
- A same-sex marriage.
- A marriage entered into prior to the dissolution of an earlier marriage of either of the parties, unless the earlier marriage is a currently valid marriage between the two applicant parties.
- A marriage between an ancestor and descendant, brother and sister, uncle and niece, or aunt and nephew, whether the relationship is by half or whole blood.
Marriage between first cousins, whether the relationship is by half or whole blood, is permitted in Colorado.