It sounds as if you are confusing impotence and infertility.
*Impotence *is the inability to have intercourse. *Infertility *is the inability to conceive children.
Impotence is an impediment to a valid marriage only if it is permanent and present at the beginning of the marriage. It is not “grounds” for nullity if it develops later (i.e., old age, medical condition, or an accident).
Infertility is not grounds for a decree of nullity, per se. Infertility *could *be cause for a decree of nullity if it were part of fraud. By that I mean if one party knew the were infertile and withheld that from the other, and the party that had information withheld from them would have chosen not to marry if they had known this information beforehand.