I scrolled through and tried to find whether or not this was addressed. I skimmed in places, so I apologize if it already was, but -- are all parties involved open to the idea of Kensi having another serious partner? Not an anonymous sperm donor, but a real partner who could be her husband and the father to her child if it's a man she gets involved with, or at least a second parent if it's a woman she gets involved with, in addition to her relationship with you and your husband?
If so, if she has that option on the table, then I think this could work out in a reasonable way for all involved. If that is not an option -- if her only choices, in the context of this triad, are to be childless forever or to have a child who does not have the benefit of a second parent (I'm assuming that if she had a child by a sperm donor, you wouldn't be comfortable with your husband acting in a fatherly role, since you've said you wouldn't be ok with her and him having an adoptive child... and I also assume you wouldn't be open to being a second mother to said child?), I can't see how that can work out, it just seems like a cruel position for someone who's stated she wants to be a mother to not get the benefit of a partner in parenting.
Either way, it seems like poly-friendly counseling couldn't hurt. If there's any question in your mind as to why you feel so strongly about this, why not explore that and try to understand it better?
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.