How effective is peeing after sex to prevent HIV and STDs transmission? If it is effective at all, is it more effective for men or women? I know someone who thinks this protects them totally from HIV and STDs.
Urinating/peeing after sex will not totally prevent you from STIs or HIV infection. While it may reduce the possibility of urinary tract infections (UTIs), it probably does not reduce your risk for most sexually transmitted infections (STIs).
It is common professional wisdom that suggests that peeing after sex can reduce the possibility of urinary tract infections. This advice is more applicable to women than men, since women are more susceptible to UTIs/bladder infections. This is true because during sex, bacteria from the rectum/anus could come into contact with the women's urethra, and the distance from the urethral opening to the bladder is shorter in women, making them more prone to bladder infections.
Urinating after sex will yield different results for men and women since men and women's genitals are anatomically different. The urethra in men serves the dual purpose of peeing and ejaculation, so there is essentially one opening. In women, since the urethral opening is separate from the vaginal opening, urinating may help prevent UTIs but will not prevent vaginal infections.
I'm not sure how well-documented this theory or "peeing as prevention" is. Peeing after sex surely can't hurt (if it does, you might have an infection!). It can help remove unwanted organisms from the urethra, which may reduce the risk of urethral infections. However, you should not rely upon this as a method of disease prevention, since condoms will be much more effective and are proven to work.