Mab refers to a "fusion of bloodlines" in announcing her decision to marry Harry to Lara. This seems strange considering that the Raith and McCoy bloodlines are already fused in the persons of Thomas and his unborn child. In fact, they were the topic of conversation that Mab changes with her declaration. But with Thomas imprisoned in Demonreach and the pregnant Justine in the wind, if Mab needs to have a backup entity with that genetic mix on hand for her designs she might think it be best to get started as soon as possible. Therefore, Mab would ideally want them consummating a marriage. I understand Jim says Harry will have no more children, but Mab doesn't know that.

The other element of timing is that the announcement comes while Harry and Lara are still shaking on a deal, or given the spark Harry feels, perhaps it was a Deal. Could the spark have constituted a magical binding and Mab recognized it for what it was when Harry and Lara didn't? Maybe that's why Mab chides Harry about selling pieces of his soul willy nilly for other people's benefit. And maybe, Mab sees making Lara a member of the Winter court, consort of the Winter Knight, as being a way to short circuit what Harry had accidentally done. Harry felt a strong fae generated compulsion to do things he'd rather not do in order to satisfy the favor Mab granted Lara. Perhaps Mab fears that the committment to save Justine (and/or the unborn child) will override some future order she gave and by subsuming that Deal into a larger formal alliance via marriage her wishes would regain priority of place.