What is the ruling on a young man marrying his fiancée, when both of them carry the sickle cell trait, with a 25% probability of their children inheriting the disease; and should he rely on Allah and marry her given his love for her character?

Preserving offspring is one of the five necessities in Islam, and one must avoid anything that harms it. If a disease is hereditary and there is a possibility of it being transmitted to children, then it is better to avoid marriage. However, if the transmission of the disease is confirmed by doctors' testimony, then marriage must be abandoned; because the Prophet, peace and blessings be upon him, said: "There should be no harm nor reciprocal harm," and because Islam encourages strong bodies. Proceeding to bring forth weak offspring contradicts this and is considered an aggression against the children. This does not contradict belief in divine decree, because taking precautions for safety is part of Allah's decree.