It is very common for men who have completed or who are in the midst of prostate cancer treatment to have a decreased interest in sex, at least in the short term.
Many factors can contribute to this diminished interest following treatment, including anxiety, depression, fatigue, or pain -- all of which are relatively common during or following treatment. None of these factors is necessarily permanent, and most men eventually return to a normal level of sexual interest following treatment.
Additionally, hormone therapy for prostate cancer acts to lower the level of male hormones or to dampen their effects. In doing so, the libido often suffers.
If your decreased interest in sex does not return to normal after prostate cancer treatment, be sure to discuss this with your doctor.