For example, lions and tigers, or polar and grizzly bears, are not the same animals, but, they have the same sequence of base pairs, so they can mate. Even if they find a way to mate, cats and dogs still can’t produce their hybrid.
Dogs do all kinds of weird stuff. They can mount and hump anybody or anything, due to variety of reasons, such as excitement, boredom, stress, anxiety, etc. Also, a dog might be humping a cat to show his dominance.
If you see your dog constantly mounting your cat, you should first discover the cause of this behavior. After that, you should redirect your dog’s attention with a toy or with some other activity.