Another “risk” is this amorphous conglomeration of emotional issues that people–fine, let’s be honest, women–apparently face if they have casual sex. In her amazing book What You Really Really Want, Jaclyn Friedman writes:
"You may have heard about oxytocin. It’s a chemical that is often released during sexual stimulation. Some studies have shown that when we release oxytocin, it can intensify the feelings (both positive and negative) we have about the person we’re having sex with. Unfortunately, this chemical response has been warped into an argument by abstinence-until-marriage advocates and other social conservatives, who claim that because of this bond, women get hurt by casual sex more than men. And the argument goes further, claimed that if women form an oxytocin bond with too many people, the effect will wear off and they’ll find themselves unable to bond properly with anyone.
Friedman goes on to cite studies that show that, first of all, it’s too little oxytocin that causes problems, not too much. She also cites research showing that oxytocin is also produced in many other situations, such as playing games, petting dogs, singing in groups, yoga, talking with close friends, and even eating certain foods.
As far as I and other sex-positive writers have been able to find, there is no research actually showing that casual sex is intrinsically harmful.