You are not radioactive from external beam radiation, unlike the situation with a radioactive seed implant. When patient undergo a permanent seed implant about 100 seeds are injected throughout the prostate gland, and they release their radiation slowly over months. These patients are radioactive for a few months. If you hold a Geiger counter up to their body you will hear a “beep beep beep”. They are advised not to hold small children on their laps for any extended period of time, and clinics sometimes recommend sleeping alone in bed.
Men who undergo IMRT (external beam radiation) are not radioactive at all. Once the radiation beam is turned off at the end of each treatment there is no leftover radiation left in the body. It is like turning off a flashlight — no light lingers around. The inflammatory effects will linger, but not the radiation. Imagine if you lay outside in the summer sun for an hour and then came indoors: your skin would turn red and stay red for days, but there would be no sunlight shining from your body. Radiation therapy is like very powerful light.
In a similar concern, your cancer is not contagious at all, through casual contact or through sexual intercourse. There is no danger in having sexual relations before, during, or after you have undergone a course of radiation. Sometimes there can be some blood in the ejaculation, but this is not dangerous either.