One in seven couples separate as a result of cancer, the survey found
Cancer patients are unable to rebuild their lives because they are not receiving the psychosexual support they are entitled to, a charity says.
Three-quarters of patients surveyed by Macmillan Cancer Support said their sexual relationships had been affected.
Most had not been offered any advice by professionals despite national guidelines stating they should have access to this kind of counselling.
The charity wants to encourage patients to speak out about their problems.
It says there is currently a silence surrounding the impact cancer has on sexual relationships.
Cath Lindley, general manager for Macmillan Cancer Support in Wales said: "We are aware that there is some good practice taking place in parts of Wales.
"However, our survey shows clearly that the NICE guidance on sexual rehabilitation services has not yet been fully implemented.
1/3 are unable to speak to their partner
Half are put off starting new relationship
One in seven couples separated permanently as result of cancer
Two thirds said their doctor had not spoken to them about this
"Health professionals must talk to patients about the impact of cancer on their sex lives at the earliest opportunity.
"People usually think of the hair loss and other side effects of cancer, but often it is how it effects a person's feelings about themselves that can be most distressing.
"It can leave people's confidence shattered, body image low, and make it psychologically or physically difficult to have sex at all."
Macmillan are funding a programme at Swansea's Singleton Hospital where a physiotherapist works with women who have had surgery for gynaecological cancers.
Dina Marie Bam concentrates on the prevention and reversal of lymphoedema, an uncomfortable and often painful condition that results in fluid build up in the limbs and genital areas and is often caused by the removal of lymph glands during surgery.
A scheme offering psycho sexual support to cancer patients in west Wales has been applauded by patients
She said: "Lymphoedema is a very visual condition. My patients experience swelling in the heels, feet, legs and genital areas.
"They may experience embarrassment because they feel 'big' and uncomfortable.
"This can affect their relationship with their partner but often women don't feel able to talk about how they feel and so it can become a problem."
Macmillan has written to the chief executives of all the local health boards in Wales urging them improve their services so that patients can benefit from what is promised in the NICE guidelines.
They also want health professionals to be given more resources to talk to patients about sex and cancer.
Deputy Chief Medical Officer Mike Harmer said: "NICE Supportive and Palliative Care service guidance does indeed suggest care should be patient-centred for all patients, which the Welsh Assembly Government would endorse.
"This will apply for all patients, and generally we would expect any individual to be able to discuss their sexual relationships with professionals if required.
"There are standards in areas such as gynaecological and urological cancers when it can be beneficial for specific discussion about sexual relationships to take place and it is helpful that bodies such as McMillan have leaflets available to support patients and professionals."
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