I'll tell you what the point is.
Masturbation is "wrong" because it serves as an outlet for sexual drive. Fornication, or marrying someone outside the cancer, is wrong because it serves as an outlet for sexual drive. Marrying within the cancer, while it can serve as an outlet for sexual drive, can be tightly controlled by every washtowel article that comes along (plus it can produce children that will themselves become witlesses).
Between bearing children entirely within the cancer, and field circus (you can't be out in field circus while masturbating), that is how the cancer propagates itself. And, if your sex drive is stifled, it makes that energy go toward recruiting yet other people into the cancer, also perpetuating the cancer. Any activity that interferes with channeling all drive into making that cancer stronger is "bad".