They will still mate along the way and in Mexico a bit, but most are not interested. Monarchs that are in diapause do not have adult reproductive organs. Females cannot lay eggs. When spring comes, the organs began to grow and there will be an incredible mating display in Mexico and mucho egg laying along the way and in Texas. They will go as far as they can producing the first generation of the year. I have seen what looked like a remigrating Monarch in late March in Maryland a couple years ago. She was flying low to the ground and very faded. Some actually do make it pretty far north before they die.
Herndon, VA (USA)
Take care of the small things....