I understand that antecedent impotence is ground for annulment in Catholic canon law. But what if the impotence develops AFTER the marriage? Would that also be a ground for annulment?
There are only three mentions of the word impotence in the Code of Canon Law:
1) In Canon 1084, section 1. This is the impediment which you mention:
Antecedent and perpetual impotence to have intercourse, whether on the part of the man or the woman, whether absolute or relative, nullifies marriage by its very nature.
2) In Canon 1084, section 2. This describes limits to the ability of antecedent impotence to nullify a marriage:
If the impediment of impotence is doubtful, whether by a doubt about the law or a doubt about a fact, a marriage must not be impeded nor, while the doubt remains, declared null.
3) In Canon 1680. This requires the annulment tribunal to consult an expert (presumably a medical expert) in such cases:
In cases of impotence or defect of consent because of mental illness, the judge is to use the services of one or more experts unless it is clear from the circumstances that it would be useless to do so; in other cases the prescript of canon 1574 is to be observed.
The word sterility, which is not precisely synonymous, is also mentioned in the Code, in Canon 1084 section 3. This states that sterility "neither prohibits nor nullifies marriage".
It appears, then, that impotence developed (and known about) before marriage, and known to be irremediable, nullifies and prevents marriage; but impotence developed after marriage does not nullify it. Why would this be?
The Catechism of the Catholic Church, in its discussion of marriage, states that "Conjugal love . ... is open to fertility" (paragraph 1643, quoting the encyclical letter Familiaris Consortio; emphasis added). That does not mean (contrary to many caricatures of Catholic family life) that fertility is required. The Catechism specifically says, in fact,
Spouses to whom God has not granted children can nevertheless have a conjugal life full of meaning, in both human and Christian terms. Their marriage can radiate a fruitfulness of charity, of hospitality, and of sacrifice.
Thus, if the intent and understanding of the spouses was originally to be open to having children, later loss of that ability does not change the validity of a marriage.
If one has gone into a marriage believing oneself (or knowing oneself) not to be impotent, but develops this debility later on, one's marriage is still, in the eyes of the Catholic Church, valid—at least to the extent that it was not otherwise impeded.
Canon 1141 would appear to be relevant:
A marriage that is ratum et consummatum can be dissolved by no human power and by no cause, except death.
So, no, a consummated marriage cannot be ended simply because impotence has developed. The marriage existed, is known to have existed in its consummation, and annulment is tantamount to saying that it never existed.
However, if the marriage is not consummated then Canon 1142 applies:
For a just cause, the Roman Pontiff can dissolve a non-consummated marriage between baptized persons or between a baptized party and a non-baptized party at the request of both parties or of one of them, even if the other party is unwilling.
There may be other grounds for annulment, for example a marriage contracted following the onset of permanent impotence is void.
1084 §1. Antecedent and perpetual impotence to have intercourse, whether on the part of the man or the woman, whether absolute or relative, nullifies marriage by its very nature.
If one of the parties knew that impotence was impending and did not inform the other before the marriage, there may be a defect of consent which would invalidate the marriage even though it could be and was consummated.
1098 A person contracts invalidly who enters into a marriage deceived by malice, perpetrated to obtain consent, concerning some quality of the other partner which by its very nature can gravely disturb the partnership of conjugal life.