Mikvah and contraception

Dear Rabbi
I recently learned that the reason that married women immerse in the mikvah every month is not for purification following her monthly cycle (as many believe) but more because of the spiritual impurity caused by her failing to conceive a baby that month. If some methods of contraception are permissible should that mean that a woman who practises one of these methods is not required to immerse in the mikvah because it is already not possible for her to conceive a baby and so does not need purification?
Dear Ruth,
The whole notion of ritual impurity is linked with the principle of a spiritual vacuum. Hence the greatest form of impurity is that of a deceased body, the soul has now departed. Similarly, when a woman gives birth she is rendered impure because of the soul contained within the child now leaving her womb. And each month the egg dispelled is a potential child, also bringing in its wake a spiritual void. All this is restored through the waters of a mikvah. Water, which is the prototype of fluid, unlike solid matter, is capable of change and does not have any permanence. Moreover, water is the life force of man without which we cannot exist. It balances the solidity of our universe and metabolises the body of man. Even as one may have become ritually impure they can change and become ritually pure again. If one’s spiritual essence has become affected as a result of a vacuum then we utilise the greatest physical life force available to us to restore oneself to their original spiritual state. I recommend the laws of family purity to every Jewish couple and give my unqualified guarantee it will do wonders for their overall relationship.