When the tip of a penis is stimulated—by bumping against a cervix, say, or the opening to a vagina (or any other orifice, for that matter)—several muscles contract reflexively. Among them are an anal and a urethral sphincter. The closing of the latter prevents urine from mixing with semen in the urethra during ejaculation. The closing of the two together—let’s let Dr. Shafik say it—“prevents leak of urine or stools” during sex.
