Males are milked of their sperm for many reasons.
Mistresses use prostate massage to force a man to ejaculate without orgasm. This leaves him feeling very much her toy, her puppet.
Some male slaves are frequently and heavily drained of sperm in the belief that in makes them more docile.
There are civilizations that use sperm for commercial and industrial purposes. The unique attributes that makes the sperm useful hasn’t been successfully synthesized. Or it is superstitiously felt that authentic ejaculate is superior to the artificial.
Where sperm is in high demand many males are installed in male milking facilities often called barns regardless of actual structure.
Milking slaves are in early physical maturity and have been found to produce large quantities of ejaculate. Many hours every day the milked slaves are bound to the machines that collect the forced ejaculations.
The slaves are well fed and allowed plenty of rest. But no leisure. The constant manipulation of their penis quickly terminates the pleasurable experience of orgasms. Impotence is not an uncommon result. Since the remainder of their lives will be spent in chastity devices this is of no importance.
After several years the sperm yield diminishes and the slave is removed from the milking barn and moved on to another form of slavery.