I also agree that she is not sexually interested in the cat but rather showing dominant behavior.
I also agree that every pet should be spayed unless your are a professional breeder.
Good luck!
LOL! I have to admit I've seen that happen one time in my life and it's pretty weird. Both the dog and the cat seemed to "enjoy" it. Your female schnauzer has more than accepted the cat into her pack. She's actually displaying dominant behavior rather than sexual behavior. It is normal for female dogs to adopt mounting behavior as a sign of dominance.
It might help to have her spayed, but I rather doubt it. Go ahead and spay her anyway though as you'll have no worries about mammary cancer, pyometra (sometimes fatal uterine infection), unwanted puppies and blood in the house when she goes into heat. There is every reason to spay unless you are a licensed breeder and expect to breed her.
One note of caution: a dog who mounts any other animal will not hesitate to include humans in that behavior. Any dog behaviorist will tell you to discourage any dominant mounting behavior whether it be with another dog, a different animal or a human. Why? Because that natural dominance can eventually lead to aggressive behavior. If she ever decides to mount a new dog acquainatance who doesn't intend to be dominated, you'll have a nasty fight on your hands.
You can usually see it coming in the dog's posture. If she lifts her head, ears and tail very high and looks intently at her potential mountee, immediately redirect her attention. You have to do the correction the second she thinks about doing it rather that waiting for her to mount up.