A cancer diagnosis can create many changes in a person's or couple's sexuality, but not many therapists or wellness providers are trained to recognize, assess, or treat these effects.  When cancer and sexuality are addressed in, say, a journal article, it is often related to cancer of the reproductive system, e.g., testicular or uterine cancer.  But any almost any kind of cancer can create sexual and relationship problems that, if addressed, may be readily resolved and contribute to a better quality of life during survivorship.
When a person receives a diagnosis of cancer, it puts them in touch with their mortality in a profound way.  While everyone knows logically that life is finite, cancer puts this fact into focus in a way that cannot be ignored.  Cancer is an existential threat that often causes worry and sadness.  This naturally sends a person's sex drive into hibernation, however, some people have a harder time shaking this feeling loose after treatment and their sex drive doesn't seem to return.
Cancer can also change a person's role.  If a person is used to being independent and functioning well, being ill due to chemo, radiation, or surgery can leave them feeling obligated and vulnerable.  The partner who is the caregiver may take easily to their duties, but others may struggle with tasks of nursing, such as changing dressings or bathing.  Once cancer is in remission, things may never quite go back to normal because generally there will be continued followup appointments and tests.  The cancer survivor may not have the same energy, and the partner may have difficulty accepting that the illness has changed their relationship.  Some couples struggle with becoming lovers again, even if desire and function are intact.
For people whose reproductive systems, which includes the breasts, are impacted by cancer the effects are more immediate.  Issues related to bodily functions, body image, hormonal changes, and sexual function can be embarrassing for either partner, causing them to avoid sex and sex-related topics.  Their relationship may become desexualized until, perhaps, one or both partners realize that something is missing, though they may not know how to reclaim it.
Most couples who want to be sexual again will find a way to do so without any help.  They may have had good sexual communication before the cancer diagnosis and have more flexible attitudes about what counts as "sex."  Others struggle a great deal.  They need to come to terms with not only their feelings about cancer, but about the changes it has brought, both in and out of the bedroom.  Sometimes partners come away from the experience of cancer and its treatment stronger, but sometimes it affects each partner differently, with one feeling that they have won a battle, and the other feeling battle weary.  These and other feelings need to be processed and worked through while making changes to the way in which they relate to one another sexually.  A well trained provider can make a great deal of difference in how well supported a couple feels as they adapt to changes both large and small in their sexual relationship.