Buddy Blank, 10-4.
Sounds like you are not alone, and thankfully this one does appear to primarily be short-term. A quick look back shows several other posts by men experiencing similar short-term effects but they reported it getting better over time. The prostate-cancer.com site lists a number of the brachytherapy side effects, including this section (copied/pasted below) titled “Sex after Brachytherapy.” The last sentence here says the pain usually subsides…
Sex after Brachytherapy
Sexual side effects are also common. There is some controversy about what causes erectile dysfunction after brachytherapy seed implants. Some researchers speculate that the long-term effect of radiation in the body can damage the nerves that stimulate erection or the blood vessels that supply blood to the penis.
Some patients who undergo brachytherapy seed implants also report a shortening of the penis. Some researchers speculate that shortening the urethra causes the penis to shorten as well. Other researchers, however, note that in external beam radiation therapy, some patients have reported penile shrinkage as well, indicating that the radiation may cause the shortening.
The prostate gland and the seminal vesicles are responsible for creating the fluid that makes up semen. Consequently, most men report a decrease in their amount of ejaculation. Men are still able to experience orgasm though their ejaculation will be reduced. about 5% of patients who have undergone brachytherapy report completely dry orgasms, while about 5% of brachytherapy patients report no change in their ejaculation. Some patients report pain with their first ejaculation after the brachytherapy seed implants. The pain, however, subsides with subsequent ejaculations.
When BT first got it's start treating PC, it was primarily for guys who could not withstand the rigors of surgery. It's grown as a "convenient" means of treating the disease, and has migrated to younger guys who have to balance this with their concerns (particularly thinking of their longer-term concerns) about potency.