There are many legal marriages that masquerade as sexually healthy, loving relationships. These marriages were probably sexual and loving early on (though some were not), but they no longer are marriages, except legally. These masqueraders are roommates, and sometimes parents, but they are not lovers.
Although there is no close estimate of the proportion of legal marriages that are sexless, my experience is that there are more such marriages than meets the eye. I would distinguish between marriages that are sexless because one or both partners choose not to have sex, and those where there are health or medical reasons for not being sexual. Many–proabably most–who no longer have sex because of health reasons can be helped to restart their lovemaking. They simply need to seek medical and mental help to do so.
Even if intercourse can no longer be enjoyed, do these spouses pleasure each other with oral and manual stimulation? How do they express love to each other? Do they affectionately cuddle and make out? Or are they entirely sexless and loveless?
A sex therapist sees those who want to fix not being fully or even partially sexual. I am convinced that most sexless couples do not seek help to become sexual again. They may barely talk, let alone make love. There may be resentments that go deep and long. If all of these sexless marriages were identified as marital frauds, there would be a lot fewer marriages, and those remaining would be true marriages where sex and love are integral to the definition of the relationship.
There are hormone imbalances which may affect sexual desire, and there are prescription drugs which negatively impact desire and arousal, let alone orgasm. These problems can usually be fixed by competent doctors and sex therapists. Resentments and low self-esteem can often be addressed by a board certified sex therapist. Similarly, stress from work and relationship woes can be dealt with in therapy.
Some marriages never were very sexual. In many of these cases, a lack of sexual chemistry (the pheromone/smell atraction) and/or sexual inhibitions from repressed family and religious backgrounds may be culprits.
Whatever the reasons for the lack of sex and love, life is too short to make believe that a marriage is healthy and viable when it is neither. Most people either take too long to get out of bad relationships, or they remain miserable in them. Why stay unhappy? It is a CHOICE to remain stuck in an unhealthy marriage. It is also a CHOICE to get out and find real sexual connections elsewhere. I vote for the second choice to move on and find playful lovemaking with another lover if the marriage cannot be sexually fixed.