What Causes UTI After Intercourse?

Hi Doctor, My husband(65years old) has retrograde ejaculation after Turp operation 5years ago. I have noticed that after sex he (a lot of the times) gets a UTI but his GP does not think that they are related. I definitely think that they are most definitely related.In the last 3 months he has suffered UTI & has been prescribed antibiotics but once he stops taking them the infection comes back again, his prostrate has been checked out & is normal,PSA is normal & Ultrasound normal,he is having a uragram done in 2wks time. I firmly believe that the retrograde ejaculation is related to his UTI s -Can you enlighten me on this subject