I believe, (I’ll try to check my sources) The basic requirement is that except for advanced age, or sterility that is caused by medical nessesity, ie a woman with ovarian cancer must have her ovaries removed. Both parties must be judged by usual means as able to concieve and bare children. So an impotent man, (before Viagra) was unable to marry, (upheld as valid grounds for an annulment if the condition predated the marriage) Or a man who was a homosexual before marriage with little or no inclination to change his behavior, was considered unable to marry, and granted an anullment.

We must remember the Annulment is granted because there existed prior to and at the time of the apparent marriage some sort of impediment to marriage.