A man fathers children with his adult step-child, is that incest by Colorado statute 18-6-301? Class-4 felony

Colorado statute 18-6-301. Incest.

(1) Any person who knowingly marries, inflicts sexual penetration or sexual intrusion on, or subjects to sexual contact, as defined in section 18-3-401, an ancestor or descendant, including a natural child, child by adoption, or stepchild twenty-one years of age or older, a brother or sister of the whole or half blood, or an uncle, aunt, nephew, or niece of the whole blood commits incest, which is a class 4 felony. For the purpose of this section only, "descendant" includes a child by adoption and a stepchild, but only if the person is not legally married to the child by adoption or the stepchild.

The law seems to contradict itself when it says "Any person who knowingly marries... a... descendant... stepchild twenty-one years of age or older... commits incest",
but later says "descendant includes a... stepchild... only if the person is not legally married to the... stepchild".
In other words it's incest if he marries her, unless he is legally married to her.
Therefore, this "law" should be considered nonenforceable as it stands, as it does not make clear any timing - for example, if he marries her and fathers her children before she becomes his stepchild.

If you know this law, and have posted it here, why are you asking.

The only thing you left out was the age of the "child" at the time. What is the age of consent in Colorado? Does the "child" attain majority at 18 or 21?

If he marries the step-child beforehand, then it is not incest.
If he does not marry the step-child beforehand, then it is incest.

Yes it does, so long as he does not marry the step child beforehand. If the stepchild is UNDER 21, more severe charges come into play. There are very few cases on this, and most of them before 1930.

That's what it appears to say, yes.