Was this posted by Stephen Fry?
I have to say the tone of the question or at least the way it is worded does not bode well. If this question is actually about the poster, I would say from your tone it is a doomed relationship. If its about someone else, you seem a bit resentful or hostile in some way. I could be misreading it though.
How does the person in the relationship personally define ‘love’. How do they define ‘happiness’ in thier own eyes, not in other peoples judgements?
If they have a great life other than sex they are luckier than many people. How would they judge things if they were reversed and they had great sex, but no home life/kids/suchlike? Life is a balancing game – you win some things but at the expense of loosing other things. There is rarely a win win. Only the people in the relationship can truly know what aspects of life they rate as the most valuable.
The person who is not interested in sex (We are supposed to assume it is a woman, though from the wording I guess it could be 2 males in a gay relationship?) – we do not know their reasons for being disinterested – physical difficulties, a horrible past experience, the present partner is not nice to have sex with for some reason, someone cheated so they dont trust them? Could be almost any reason. I would suggest the couple ask to go to either relationship or sex counselling to find out if anything can be of assistance mentally or physically to either of them and to help them find out what (if anything) is wrong, and what they both really want from life. Sex and relationship counsellors are very non judgemental and have probably heard every bizarre behaviour around, so they can have a lot of constructive suggestions.
Love is more than sex. It is complex and often bizarre to outsiders, but of huge value to those involved, even if no one else understands their association.