They should sell sterilized semen in the supermarket.
I have never cum into a vagina unprotected. One girlfriend I had was on birth control and she still made me wear a condom. I think the idea of having a baby with me makes them want to double up protection.
Some dude commented this... I guess it could be true.
Actually, semen contains E and F-type prostaglandins, which are addictive neuromodulators (makes you happy). On top of that, orgasm releases Opiates (natural form of opium), oxytocin, serotonin and several endorphins in the brain, the same chemicals released when you shoot up heroine, so of course sex is chemically addictive!
The average load of Semen also contains 3,000 mg of the amino acid L-arginine, as well as 130 grams of protein.