Mounting behavior is not necessarily sexual in nature. Even female dogs will mount males or other females. Why? Because it is a dominant behavior. When a dog mounts a human's leg for instance, it isn't due to sexual excitement - it's because the dog is "claiming" the human as a lower member of its pack. Either way, your male dog should be neutered, and if that doesn't calm him down, you need to consult a veterinary behaviorist to help you learn to be the Pack Leader.