In my experiences an un-neutered buck will hump everything and anything until castrated. I'm pretty sure if you have Midi spayed she would still have a lot of unwanted attention from Dexter, which might cause problems with them getting alone if Midi gets fed up with the humping she might turn on Dexter. If Dexter doesn't hump Midi them go for it just be wary that the phrase "humping like rabbits" didn't come from no where.

I know when I first got my Charlie until he was castrated I let him release his frustrations on a soft toy rabbit so my hand was safe from being humped when trying to give him his dinner.