Yet many patients could be helped if more services were available on the NHS, it said.
Erectile dysfunction is a potential side-effect of surgery, radiotherapy and hormone therapy used to treat the condition.
More than 40,000 men are diagnosed with prostate cancer in the UK each year.
In some there may be permanent nerve damage, meaning they cannot maintain an erection.
In others the physical problem can be temporary, while for some the treatment has led to a psychological barrier to sex.
Two in three prostate cancer patients said that they are unable to get an erection.
Macmillan said that men also needed to feel they could seek help when they were having problems with sex after their treatment.
The charity wants to see specialist nurses, better psychological support and physiotherapists more widely available to prostate cancer patients.