Did Parker Townsend and Christina Reese get married in the Thoroughbred books?

No. Christina is about 18 or 19 when the books end, and Parker would be 20 or 21. The only update we hear about them is that Christina is still perusing a career as a vet and Parker is accepted to USET, so he's Olympics bound. http://whitebrookfarm.blogspot.com has recaps of the Thoroughbred books, including the last few, in detail.