The Code of Canon Law states:
Antecedent and perpetual impotence to have sexual intercourse, whether on the part of the man or on that of the woman, whether absolute or relative, by its very nature invalidates marriage (canon 1084 §1).
If the impediment of impotence is doubtful, whether the doubt be one of law or one of fact, the marriage is not to be prevented nor, while the doubt persists, is it to be declared null (canon 1084 §2 ).
If it were doubtful whether a disabled person could consummate the marriage, or if it were believed that the injury could be reversed, the marriage could proceed. Since the person of whom you heard was denied marriage, my guess is that his injury was such that the marriage cannot be consummated and the inability to consummate the marriage is believed to be permanent.
Please note that impotence is not the same thing as sterility. Sterility on the part of either the man or the woman is not an impediment to marriage (canon 1084 §3). What is at issue is with whether or not the marriage can be consummated (a necessity), not with whether or not it is possible for the marital relations to produce children (not a necessity, although the couple must remain open to its possibility).