Two things I can confirm from experience (high dose rate brachytherapy boost 12 years ago plus lots of hormone therapy):
Your 'old fella', even if it does not get stiff, still has all its feeling, and orgasms still happen - by self or by partner stimulation;
Your partner likely still has a need for attention and intimacy, and will likely be happy to try new ways that you can mutually pleasure each other sexually.
I suggest you seek advice as a couple, before treatment if possible.
Prostate Cancer Foundation of Australia (PCFA) has a free advice service.