In the D/s relationships I've had, my girlfriend has milked my prostrate. In the BDSM community, prostrate milking is used to release the seminal fluid without allowing a man to have a pleasurable orgasm. If you massage a man's prostate, built-up semen will be released, but the release will not be intense. The semen kind of just flows out in a long stream, instead of shooting out like during an orgasm.
In all reality, it is prostate massage, but it's not really of a pleasurable nature. Hence the "milking" aspect. You're really just milking the semen out of him rather than allowing him an orgasm. Prostate milking is used in the BDSM community as a means to keep submissive men constantly aroused (and submissive) while keeping their prostates healthy. Prostate milking is often used as an orgasm denial and humiliation tool.