Cave and gape are two slang terms used to describe any bodily orifice that is open either for or as a result of sexual penetration. These terms are most commonly used to describe a dilated anus or vagina. Cave and gape are common adjectives used in the adult film industry.
Kinkly explains Cave and Gape
The terms cave and gape are often used as descriptors in pornographic films. For example, a film featuring anal sex might describe a woman’s "ass cave" or "anal cave." Anal sex may also make a woman’s asshole "gape like a cave." The terms can also be used to describe the vagina’s condition after vaginal sex when the woman may be described as having a "gaping cunt cave."
A person can keep their rectum relaxed and dilated for some time, through a process called anal training. With this practice, the anus can accept penetration easily and without discomfort. This skill is particularly useful for gay men who tend to bottom or submissive partners in dom-sub relationships. A butt plug is often used to assist anal training.