Though promiscuity is certainly not frowned upon in magick, there are still valid reasons to practice chastity such as discipline and temporary denial to make the pay off even more delightful. For sex magick a brief period of chastity is always beneficial.
It really boils down to immediate pleasure vs. the buildup of future pleasure. To get a feel for this sensation, try sex games that include some sort of delayed gratification, from a few hours of “torment” up to weeks or months. The more pressure that builds, the greater the climax, which is especially important in sex magick. While there are usually diminishing returns after an extremely long time without a climax, the general rule is that the longer one can be chaste, the more powerful the resulting orgasms. These exceptionally powerful orgasms can be effectively channeled into your magickal goals. Imagine how much more effective the magickal spell infused from a Lingam with a month of stored up energy as opposed to just a single day.
So while most modern magicians are the opposite of prudes, the more successful practitioners of sex magick usually include some sort of period chastity, at least before major rituals.