Is refusing to complete a marriage due to thalassemia test results showing that both the suitor and the fiancée are carriers of the gene, with a 25% chance of their children being affected, considered a transgression against Allah's divine decree and destiny?

Your marriage to this girl does not contradict acceptance of God's decree. However, it is preferable for you to seek another woman who is not a carrier of the disease to avoid having children afflicted with it, as the probability of infection is 25% if both spouses are carriers. Ensuring the health of one's children is an important objective when choosing a wife.