If your prostate is swollen enough to cause issues, it may be time to use a bike that sits you more upright, maybe even a recumbent.

Much of the time, pain and numbness are due to saddle adjustment -- if the nose of your saddle is tilted downward, you may be sliding forward and putting pressure on the wrong areas. Even if the bike is set up perfectly, it's often a good idea to spend a little time out of the saddle now and then to keep blood flowing, and shift the load to different muscles.