See, illness, circumstance, etc aside. I don't think it's fair no, to just decide on behalf of someone else their sex life is over forever, no discussion.
I am a monogamist, I do believe in monogamy too. But I think if you relationship is at the stage no intimacy is happening, just because you can't be bothered or don't "feel" like it, then the marriage is in trouble.
Yes there is more to marriage than sex, blah, blah, blah. But I don't think than anyone person has the right to dictate or enforce a celibate life style on anyone at all. Sex is a big part of a lot of peoples lives, it's nature. Intimacy with your partner is natural.
I think is someone decides, or feels, "that's it no more sex for them". Then the other partner has several choices, accepts it, leaves or comes to an arrangement.
I don't think anyone has the right to say to another human , "I don't want sex, so that's it for you, as I don't want you going anywhere else either".
That's not love, it's control. You can't tell someone they have to remain in a celibate relationship, change the goal posts and just expect them to do it. They have the right to object about them going elsewhere, if that's what they chose. But then the partner who is being told they can't have sex, also has the right to object and walk away from enforced celibacy.