L.M.
There are deeper issues at work than his mother getting time with your kids. First of all, I have never understood why some family members seem to only be interested in spending time with the kids in the absence of the parents. Particularly when they haven't spent enough time around the kids to learn the routines, and develop a comfort zone with the kids. Big issue for me! (I'm lucky, my husband is on board with me on that one.)
You and your husband need to work on your marriage, but don't let that serve as a gateway to being railroaded into letting his mother do whatever she wants with your kids to give you "couples time."
Best wishes to you.
**** I just want to add that it is poor logic to say a grandparent would be good with your children because they successfully raised your spouse! I know many people (my spouse included) who would be the first to tell you they are the person they are *in spite* of their upbringing, not *thanks to.* Raising kids is a complex symphony of many influences. As the parent, you are the conductor and should not feel pressured to change the tempo if it goes against your instincts.