I'm still a relative post radiation newbie, less than one year out, but that is not true here - at least not now (and for me things were good at first shortly after, then not great for a period, now pretty good again). You can find surgery patients who have recovered well sexually, and can find radiation patients who have not recovered well sexually, but my reading of the studies led me to believe your chances are better with radiation - one of the reasons I chose that route.
Radiation kills the nerves and erectile function will permanently cease. If you have a good surgeon erections can return eventually.
Age 61, Fit
-DX Jan 2015
-PSA (Finasteride reduced) 2/15 - 3.46, 9/14 - 3.38, 8/14 - 2.5, 2/14 - 2.1, 9/13 - 1.7, 12/12 - 1.4
HDR Brachytherapy at UCLA 4/30 and 5/7
-2 cores positive - 30-40%
-Gleason 7 (3+4)