The urine of a healthy person should be sterile. It is filtered by the kidneys, and the length of the male urethra means that the flow itself tends to make it more difficult for alien organisms to travel back up into the bladder. Females are more susceptible to UTI than males because the female bladder is much shorter, and other reasons such as the proximity of other bodily orifices and sexual activity.

Anything backflowed into the bladder that is not sterile can result in a urinary tract infection (UTI) which can lead to a kidney infection, which can become chronic, and can ultimately lead to kidney failure. I don't think any medically qualified person would endorse the kind of activity you suggest.