Marriage requires at least
the ability to have unitive sex, and no deliberate hindrance of procreation. You don’t actually need to be fertile, just not deliberately hinder it. It’s because of the fundamental Christian concept of forgiveness that the Church allows one who has had a vasectomy/hysterectomy by choice to be married. If the person sincerely repents, then the Church shows her mercy. If a man is permanently impotent, however, then the most basic act which distinguishes a marriage–conjugal relations–is impossible. There is no point for having a marriage at all. A man and a woman who love each other but cannot engage in marital relations due to physical impediments can simply live together ‘as brother and sister’ if they so wish, and this does not exclude them from the other Sacraments.
The thinking of the Church is: if you
physically cannot have sex, then there is no Sacrament. Remember that the physical sign of the Sacrament must be present in order for it to be valid. In the case of marriage, this sign is the conjugal union of a man and a woman. If a couple is not deliberately infertile (or if previous acts are sincerely repented) then a truly unitive, conjugal union can take place and the Sacrament is present. This simply cannot happen in the case of permanent impotence. Thomas Aquinas explains the difference from those who vow continence (such as the Blessed Virgin and St. Joseph) since theirs is a voluntary act, and the *possibility of *conjugal relations maintains a valid Sacrament. Again, not the case with permanent impotence. Like Nonie said above: you can’t give what you don’t have. Likewise you can’t promise *not *to give what you don’t have anyway!
Also, many Catholics are led to believe that the invalidity of homosexual marriage is simply that it is not able to procreate. That reasoning is a little faulty/incomplete, as we can see on this thread, since an infertile man and woman are also not able to procreate. The invalidity of homosexual marriage again lies primarily with the nature of the Sacrament–the conjugal union of one *male *and one *female. *Homosexual ‘marriage’ is not sacramental because of the lack of the proper physical sign. It is neither procreative nor unitive because it is unnatural (as in, contrary to Natural Law). The unitive aspect doesn’t take place either because there is no natural union. The ‘key’ does not fit the ‘lock,’ because either the ‘key’ or the ‘lock’ is missing