Hello Alan,
Sorry for your diagnosis news, and that you find yourself here. However lots of very helpful guys with past experiences to call on.
I had 3 weeks of EBRT followed by HDR Brachytherapy. My cancer wasn't located where yours is. However, the consent form I signed listed (among other things) the increase in chances of ED from 10%-50% to 50%-100% and of Urethral stricture 1 in 100 to 1 in 10 (or around that).
I guess its the strictures that could be more of an issue for you being where your cancer has been located. I was given the sign off sheet to take home so I could fully read and digest the implications so that might be worth an ask along with asking for a second opinion which would be a wise plan.
These are issues that can affect you months or years down the line, I'm only 7 month on, but so far so good. Not sure if being older or younger is a good thing here. I'm 49 maybe more time for issues to develop?
All the best John