Is it dangerous to have anal sex after having radiation therapy for prostate cancer?

No. The rectum is somewhat irritated during and shortly after radiation, so use of lubrication and common sense is recommended. If it hearts and is too irritated avoid till symptoms resolve.
Sex. Prostate cancer radiation may scar the anal sphicter and also make the surface lining of the rectum more friable or fragile. Take it slow ad use plenty of lubrication.
It can be. There is fibrosis and mucosal changes after xrt, these can lead to pain and local tissue damage. It is also postulated as part of the reason homosexual men report greater debility and decreased quality of life following treatment.
Possibly. If the receptive partner has had prostate treatment (or anal cancer) there are risks. Radiated tisues are less pliant, and he anterio rectal wall has received quite a bit of radiotherapy treatment. At 3 years, about 5% of people suffer rectal bleeding form radiotherapy induced rectal injury. The active partner should not have any post radiotherapy effect as long as he is interested and potent.
Here are some ... Any forms of sexual acts should be okay for a man after RT (radiotherapy + seed implant) as long as being comfortable & tolerable. Remember, as we age, all the tissue, especially after RT, will not be as elastic and strong as in young age; so, apply commonsense & use more lubricant but still watch for injury and possible contracting unwanted STDs. More? Ask Doc timely for more counseling.