From a story standpoint, I thought it was brilliant and funny.
From Mab's standpoint, an arranged marriage for her Knight to form an alliance makes perfect sense. Historically, most monarchies in every part of the world used marriages to signal the creation of a new alliance or strengthen an existing one, and Mab is the right age to think the same way as the decision makers of those old monarchies.
For Mab, it's a political move and as Bigdangmoose said, it could be a way to sink her hooks deeper into Harry, Molly and Lara. Also, Mab might not care if the marriage is ever consummated. That had to happen to seal the deal when Harry became her Knight, but I'm not certain if a political alliance needs to go that level of commitment. Obviously, Mab doesn't want Lara to get control of, or devour her Knight, so a sexless marriage might suite Mab just fine. However, such a marriage would still have all of the negative associations with Harry's current friends and former friends and allies in the White Council. Would anyone believe that Harry and Lara Raith had a platonic relationship? Imagine Harry trying to tell Ebenezer that.
My guess is after Mirror Mirror, Lara will be Harry's sidekick in the book that follows, though I doubt this will be a regular occurrence. That's the book about retired gods; maybe Greek gods, who work as professional wrestlers to keep people; their fans, worshiping them. Lara will still have a vested interest in protecting Harry because Thomas will still be stuck on Demonreach. So Harry might work undercover as a professional wrestler while Lara pretends to be his manager. They will bicker constantly and the people they meet will ask them how long they have been married, while the wedding Mab is planning for them is days away.
Long-term, even if their was some way for the two to consummate their relationship without Lara killing or enslaving Harry, and assuming the love protection is broken so Lara doesn't get burned every time she touches Harry, they don't make a believable match. For that to happen Lara would have to go through a redemption arc and I'm not seeing that as likely. Long-term, Molly is the better match for Harry, if Molly can become human again, and that's a big if.