When does pre-cum come out of a guy's penis?

Pre-ejaculatory fluid is secreted by twin structures called the Cowper's glands. These glands are tiny, and they are located at the base of the penis⁠ (penis: One of the two external reproductive organs of people often assigned male at birth.) . The fluid they make lubricates the urethra⁠ (urethra: The urinary tract, where people pee from. On a penis, the opening to the urethra is at the head of the penis; on a vulva, the opening is above the vaginal opening and below the external clitoris.) and facilitates semen⁠ (semen: Fluid which comes from the urethra of the penis during ejaculation, and which usually contains sperm.) flow during ejaculation⁠ (ejaculation: In a sexual context, a discharge of genital fluid, usually (but not always) as a result of sexual stimulation and/or orgasm.) . Pre-ejaculatory fluid starts to seep out⁠ (out: Short for ‘out of the closet’. When someone’s LGBTQ+ identity is known to other people.) during sexual⁠ (sexual: About or relating in some way to sex or sexuality.) arousal⁠ (arousal: A state of sexual excitement and interest that sends messages to the brain which create physical changes and sensations, such as increased blood pressure, erection, lubrication, loosening of the vaginal or anal muscles, and increased physical sensitivity.) , so it comes out when he's erect. Sexual activity is not necessary to initiate the secretion of the fluid, but it will continue during the sexual activity. Some guys will secrete more fluid than others, so if a guy doesn't seem to be releasing any fluid, it might just be that he's not releasing much fluid. Other guys might release a flood of pre-ejaculatory fluid (though generally more semen is produced than pre-ejaculatory fluid).

For more info about male anatomy⁠ (anatomy: The body, parts of the body, or physical structure of organisms like people, animals or plants.) and function, check out Innies & Outies: The Penis, Testes, and More.