that he is able to have intercourse at times, but unable to do so at other times?
Answer: This is possible, but he is not considered impotent, because it is not permanent.
Can he be impotent with respect to one wife but not impotent with respect to another?
Answer: In reality, this is possible—especially if, Allah forbid, he has been affected by sorcery. There is sorcery intended to induce affection and sorcery intended to turn people away. Thus, he may be entirely unable to have intercourse with one particular woman, while being able to have intercourse with another. With regard to the first wife, the position of the madhhab is that she has no right to annul the marriage, because he is not impotent: he is capable of intercourse. The correct view—and the one I consider preponderant—is that she does have the right to annul it, for a ruling depends upon the presence or absence of its effective cause. Indeed, it could be said that she has a greater right to annul the marriage than any other woman, because she sees him with her co-wife, bathing three times a day, while he does not come to her. This is more distressing for her than if she were his only wife. Therefore, she should be allowed to annul the marriage out of compassion and mercy toward her. Perhaps Allah will make it easy for her to find a husband through whom she will have what suffices her.
“And if she admits that he has had intercourse with her, then he is not impotent. And if she says at any time, ‘I accepted him as impotent,’ her option is forfeited forever.”
His statement, “And if she admits that he has had intercourse with her, then he is not impotent,” means even if it occurred only once. Thus, if she says that he had intercourse with her on the first night, then he is not impotent, she has no option, and she must remain with him for the rest of her life, enjoying neither marital intimacy nor children. We would say: Be patient; perhaps Allah will grant him strength, or he will die, or he will divorce you—the matter is in Allah’s Hand.
Accordingly, according to the jurists, impotence does not arise after the fact: once a man has had intercourse even once, he is not considered impotent. But this view is weak and contradicted by reality, for impotence undoubtedly can arise, since a person is liable to lose his faculties.