I've never seen a scientific journal claim this, but maybe someone else has? Here is what a quick google search turned up.
http://www.free-press-release.com/news/200711/1195498588.html
United States of America (Press Release) November 19, 2007 -- The benefits of loading semen on the skin has been discussed many times. While no scientific research has yet provided the definitive answer on the benefits of applying sperm to skin those who practice its application claim it gives them a softer more healthy complexion.
There are many active ingredients in sperm. The average man produces about 3.5 mL (1-2 teaspoons) of semen per ejaculation , which contains:
water
blood-group antigens
ascorbic acid (vitamin C)
chlorine (natural cleaning agent!)
calcium
pyrimidine, purine, pyruvic acid
citric acid, cholesterol, creatine, fructose, deoxyribonucleic acid, glutathione, inositol, nitrogen, hyaluronidase, magnesium, lactic acid, phosphorus and potassium
sodium (it's good to have, if you've been sweating a lot)
prostaglandins
vitamin B12
spermidine, sorbitol, spermine
zinc
It doesn't look like a good moisturizer. Uric acid, salt,alcohol, chlorine and zinc are not exactly known for making your skin smooth and soft. It's about 95% water, with the rest comprised of sperm, few trace nutrients and sugar, among them, zinc and vitamin C. There is no research showing that semen is good for skin, but it is not harmful either.