What are possible reasons that my boyfriend has to pee after sex?

May Be Prostatitis: When the prostate is susceptible, ejaculation can result in irritation to the bladder. Urgency/frequency of urination and a variable stream can result. While harmless if that is his only symptom, it could cause other painful symptoms. He may also be of the habit to urinate after ejaculation. Some men do so to clear out residual semen that may otherwise leak out into the underwear.