In a pure technical meaning, is the competition of various sperm, to fertilize a female. It is also a term used to denote why some males engage in watching their partners cheat, as a form of stimulation, arousal.
Scientific studies have shown that sperm actually competes for the ability to travel through the female organs, to fertilize the eggs.
In some instances, it has shown that those who have multiple partners, actually have larger testes.
Gorillas for example, are monogamous, and have smaller testicles, for producing sperm, than males do.
However the Bonobos Chimp which is not monogamous in its reproduction cycle, has larger testicles than males.
This is due to the copulation not being restricted to just one male partner, by the female gender of the species.
Practice (Associated Acts):
It is believed, that the need to produce more sperm, than would be needed for having just one partner, forces the body to adapt, to grow the testicles larger, for producing more sperm.
In brief, it appears that those who engage in sexual intercourse, with multiple partners, who require more sperm, will have larger testicles.
Sperm Competition is considered a reason for the sexual fetish, Cuckold.
It is considered a biological urge, impulse.
Mostly accounted for by the impact of sexual arousal, on the brain.
It is the mental image and natural desire to compete, that allows for many to participate in swinger parties, watching their mate (spouse) having sex with a stranger, or other partner.
This image, creates an urge to compete for the affection of their spouse, to assert their superiority.
The belief is that in order to compete, to remove the competitions sperm, or the spouses, the intercourse will be more intense, with deeper thrusts, longer periods of penetration.