Human participants in development of a hybrid species assume that their eggs and sperm are taken so that the desired Zetan genes can be mixed in during conception. Often female participants assume that they have been impregnated via artificial insemination, when in fact they have had the fetus reinserted for a brief gestation period. In the main, mixing the egg and sperm is the process, but several steps are involved.
Genetic engineering by arranged marriages has not and will not occur as part of the hybrid program. Our hybrids participate in the hybrid program much as human participants do, by making contributions. At a much later date marriage and the natural offspring will of course be the means that the hybrid race continues, but it is not yet ready to be put on automatic pilot, as you say. The small amount of Zetan genetics that is mixed into the human population, as a result of contactees volunteering to help us gather data or requesting modifications to help them in their work, does not result in hybrid offspring. These genetics are local modifications, affecting the lining of the digestive tract, the brain neurons in a certain part of the brain, or the ability of a gland to produce a slightly different enzyme. These changes do not affect future eggs or sperm that would be produced by the contactee.