It is not harmful, it is perfectly safe. Some call it the love potion. Semen is only one percent sperm; the rest is composed of over 200 separate proteins, as well as vitamins and minerals including vitamin C, calcium, chlorine, citric acid, fructose, lactic acid, magnesium, nitrogen, phosphorus, potassium, sodium, vitamin B12, and zinc. But the woman should enjoy and do oral sex or swallowing semen as for many women it is an understandable aversion (as urine also comes from the same hole). Few woman don't mind and enjoy, no harm. All the best.