The article doesn't say anything about erectile dysfunction following
treatment.  It outlines the theoretical reasons why proton beams
should produce less damage to surrounding tissues than X-rays, but
apparently the observational evidence doesn't show any such advantage.

Generally, radiation doesn't produce immediate impotence, but it can
arise later, as much as two years or more later.   With surgery,
almost everyone is impotent immediately after, but many regain
erectile function from 3 months to a year (sometimes two or more)
afterwards.

For younger men, a skilled surgeon can prevent permanent impotence in
80 percent or more of  patients, which is as good as radiation can do,
in some cases better.   For men past 70, even the best surgeons can't
do better than about 25 percent.   On the other hand, radiation can
preserve erectile function in as many as 50 percent of such patients.
Also, for radiation patients, often Viagra will work in overcoming
impotence.

So it would seem that if you are younger,  you shouldn't let worries
about impotence determine how you decide, bu if you choose surgery,
make sure your surgeon is skilled in the procedure and ask about past
results for men like you.   If you are older and concerned about
impotence,  choose radiation.

But keep in mind that whatever you do,  impotence can be treated, so
don't give up on the prospect of a sex life.   I like to give my case
as an example.  I had an RP at age 67.  With my surgeons, the odds
were about 50-50 that I would be permanently impotent.   I was
impotent for 18 months, during which time I use a pump, and while not
ideal, my wife and I were able to continue much as we had before
surgery.  I regained erections after 18 months.  Now, at age 75,  I
have nocturnal erections, and I manage during sex using Viagra,
although on occasion, if conditions are just perfect, I can manage
without it.   But we could have gone on indefinitely  using the pump.

Many men have similar experiences using injections, which sound scary,
but apparently are not difficult to self administer.  There is often
some pain a couple of hours later, but I understand that with time,
that goes away.   Injections have some advantages over the pump, so I
think I would have tried them had the issue not become moot.