A significant side effect is that you are infertile, and if you are at the younger end of the prostate cancer age range, that may be a concern. Otherwise it might be a BOGOF vasectomy, if you hadn't already had one. (Note that in the case of radiotherapy, there is still a chance you might generate some semen with viable sperm, so it's not necessarily a guaranteed contraceptive.)
A second side effect is I no longer buy Kleenix in bulk from Costco.
Orgasm can generate some considerable abdominal muscle contractions, and these might show up any internal scars or adhesions as painful.