Chris, the only reason that many surgeons advise their RP patients not to ride a bike for a while after the op is to give the nerve bundles the best chance of repairing and erectile function to be regained. Even then, not all surgeons believe it makes a difference and those that do seem to vary between advice to wait 3 and 6 months. Mr P asked John to stay out of the saddle for 6 months so he did.
There is no research to suggest that cycling can spread or aggravate prostate cancer cells. In fact, the opposite could be true - a large piece of EU research with professional cyclists found that although they have slightly elevated PSA readings, they are no more likely to develop prostate cancer than any other male. Cycling is a great way to keep fit - go for it. If you are concerned, treat yourself to a prostate-friendly saddle, available from most cycle accessory shops.