Sperm is made of chromosomal units and has a little but of glucose/carbs in the tail to give it energy to run. The base that sperm is swimming around in, is actually less acidic than a vagina, so that the vagina doesn't kill off all the sperm. This liquid also has a little bit of proteins and energy-related components.
In all honesty, it's probably a lot like putting egg whites on your face. Eggs are also pH basic.
I'm pretty sure that's the reason some women use breastmilk for skincare too - lots of antibodies, neutral pH, and nutrients.
If you've ever gotten, ahem, bodily fluids on skin, more than likely the glow you got was more from the activity itself, since vigorous activity pumps blood everywhere, including your face. That and sweating, endorphins, etc, make you look pretty darn good