Casual Dating and Hook-Ups: Antiquated social practice? Sluts on the loose?

Have you ever found yourself looking at someone and thinking “Damn, I’d totally hit but not commit?” “Ready to bone but stay alone?” “All up for mating but definitely not dating?”  or something along those lines. [It really didn't take too long to come up with these, surprisingly.] Why is everyone so hellbent on being monogamous?