NOTE: ONLY APPLIES TO DISSOLUTION OF MARRIAGE AND LEGAL SEPARATION. Spouses seeking a Colorado annulment must meet one of the very specific statutory grounds to receive a declaration of invalidity of marriage.
Colorado divorce law is no fault - either spouse can petition for a dissolution of marriage (the legal term for a divorce in Colorado) or a legal separation simply, and only, because the marriage is irretrievably broken. C.R.S. 14-10-106(1)(a)(II). The old grounds of adultery, cruelty, desertion, etc. have been abolished, and likewise the old defenses to divorce, such as condonation, insanity or collusion, have been abolished. C.R.S. 14-10-107(5).
Interestingly, adultery is still technically prohibited in Colorado, per C.R.S. 16-6-501, however no penalty is defined for it, and it appears to have been last prosecuted in 1925!. And if one spouse is in the military, then adultery is still a violation of Article 134 of the UCMJ (see Manual for Courts-Marital, Part IV - Punitive Articles, Para. 62 for more information). However, the fact that conduct may be an offense in the military does not make it relevant to a dissolution in Colorado.
Since Colorado is no fault, Colorado family law courts will "keep it clean", and not even allow a spouse to put on evidence of wrongdoing by the other spouse, with only limited exceptions (e.g. abuse may be relevant on the issue of parenting, or wrongfully disposing of a marital asset may be relevant on the issue of property division).