Colorado Statute says it is illegal for any married person to have sex with any other person other than that person's spouse. It applies to any marriage, regardless of separation until that marriage is legally ended in death or a final divorce.
That said, the statute does not define a punishment for violating this law. It has been many years since anyone has been prosecuted under the law. There is an effort to repeal it, and a recent US Supreme Court decision brings into question whether such a law is even constitutional.
So while the statute would cover a separated couple, I would not put much weight in the chance of a successful prosecution under the law.
I hope this helps!
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