This is indeed a very complex issue and I think the answers depend very much on the circumstances.
If the partner who does not want sex is ill then I think the healthy spouse really has to accept that sexual relations between them may no longer be possible. Through the ages men and women have found themselves in this difficult position and have dealt with it, either by finding a lover or mistress or by getting on with something else.
If the unwilling spouse is no longer capable of deciding for themselves due to dementia then I would definitely term it sexual abuse if the other spouse forces himself upon her. (My imagination boggles at the thought of a woman actually being able to force herself upon her unwilling husband, so forgive me my use of pronouns here).
Frankly, a permanent refusal to have sex with ones spouse signals a breakdown of the marriage or a physical or mental issue such as impotence in the man or frigidity in the woman. If both parties are compos mentis it must be up to them to try and find out how their marriage can continue in these circumstances.
Don't get me wrong, please. Some couples can live in what used to be called a platonic relationship (one without sex) and be happy in it, and that is fine by me if it genuinely suits them both.
Most of us have experienced that either we didn't particularly feel "like it" when DH did, or the other way around. If this only happens occasionally when the one who didn't feel like it was tired or off colour, well and good. If it happens more often, it may well be time to sit down and try and find out what has gone wrong, or is going wrong in your marriage.
Legally and religiously one of the reasons for getting married is the expectation and desire for a regular sexual relationship with ones partner. I remember promising to love and cherish my husband in sickness and in health, and although there are definitely other ways of doing so than through sexual intercourse, I am still fairly certain that it actually comes into it, somehow and somewhere.
The pre 1969 (England) or 1970 (Scotland) divorce laws permitted a spouse to sue for restitution of marital rights before proceeding to suing for divorce, and in anyone's book that meant sexual intercourse, so there again the presumption was that sex is a normal part of marriage. Although those of us who were teenagers in the 1960s are probably rather surprised to learn that our parents and grandparents even thought about it.
So Miss Adventure, as you see, I don't think it is acceptable for someone to go on saying no, unless he or she has explained why and OFFERED TO SET THE SPOUSE THEY NO LONGER DESIRE FREE either by divorce or the old fashioned manner of "looking the other way" when their spouse finds a willing sexual partner elsewhere.
Sorry if I am offending you, ladies, but that is the risk we run with this kind of topic.