Depriving partner of sex

I'm wondering what everyone thinks.

Hypothetical...a married couple has been together for a while , and their sex life started off normal , and then over the years one partner got less and less interested in sex until it went down to ZERO interest. This parenter turns down advances from the other , including oral sex or other forms of sexual touching. They express that they simply no longer desire sex. You dig for some more answers and that's all they can tell you. They have testing done and everything is normally medically and hormonally. This person will not compromise and will not have sex, but wants to stay in the relationship. Says sex should t end a relationship or be a big deal in a relationship. (To add let's say for the hypothetical that this issue has been going on for about 5 years now and the partner uninterested in sex also will not go to couples therapy)

Is it unreasonable for the partner who has a normal sex drive to be upset and possibly want to explore an open relationship?

Does your answer change if it's the wife not wanting sex anymore ? And if it's the husband not wanting sex anymore ?