Why is Shabbat the best time for man and woman to be together in a holy union when in fact it should be asure (prohibited) a child may in fact be concieved and that is the ultimate act of creation.

What is forbidden on Shabbat are the 39 labors involved in the building of the Tabernacle in the desert. The Tabernacle [=Mishkan] is seen by the Medrash as a miniature world; everything in the world has its parallel in the Mishkan. The building of the Mishkan through these 39 labors is paralle therefore to the creation of the world. Procreation is not one of these 39 labors.