Would a young man be sinful if he does not marry, while suffering from a congenital sexual defect that makes marital duty impossible, and its treatment requires a long time and expensive costs with no guarantee of recovery?

Marriage is not an obligation in principle. And whoever refrains from it due to an excuse, such as an illness that prevents intercourse, there is no sin upon him. Rather, refraining from it might be the most appropriate course of action. If the defect prevents intercourse, the sick person is not allowed to deceive; however, if the woman accepts his condition, there is no harm in the marriage. And it is obligatory to cut off relations with foreign women (i.e., women who are not mahram).