I understand the reasoning, but I've seen no evidence that riding can
CAUSE prostate problems.  The wrong saddle can put pressure on the
pudendal nerve and sometimes compromise blood flow.  Here the potential
problem is more of numbness and tingling of the testicles and penis.  I
have heard reports (years ago one of the editors of Bicycling magazine,
Ed Pavelka wrote an article about this) that some of the neurological
problems have at least the potential of becoming permanent.  I would
also say (with no professional expertise here) that if you DID have
prostatic enlargement, riding would certainly have at least the
potential to irritate the area and make symptoms worse.
    However, saddle technology has improved very significantly in the past
20-30 years.  I don't ride much anymore (but did in the past and hope to
in the future) and I can tell you that many saddles are made with extra
padding under the ischial tubercles, and with a cutout section in the
perineal area designed to put less pressure on the nerves and blood
vessels (as well as the prostate.  You may need to try a few saddles to
find the one most comfortable for you.
    Positioning of the saddle is also important.  Generally the nose of the
saddle will be slightly elevated.  This doesn't sound intuitive, but if
the nose is pointed down there is a tendency to slide forward onto the
nose, which you do not want.

Good luck,
Steve (former president Long Island Bicycle Club)