Here's the thing about casual sex: If you are informed and educated, healthy, your partner is informed, educated and healthy, and you both want it, then there is nothing wrong with casual sex.
If you and your partner are informed and educated, you will know not to drink or use drugs beforehand, to use birth control including but not limited to a condom (male or female, in case of accidental disease spreading), and not to expect anything from the other person afterwards except maybe another offer for "consequence-free" sex. If you are both healthy and mentally stable and use birth control there will not be any consequences. Thus, if that's the case, more power to you.
BUT. How many people, especially young people, drink or otherwise impair their judgement before having sex with someone they don't know well? How many don't understand birth control, or don't like how condoms feel? How many never check for diseases or know they have them and have sex anyway because they are impaired or they don't think they can transmit them via whatever they're doing? And how many go into it hoping that it will turn into something more long-term and are hurt when the other person doesn't call, or are having sex because they were abused or hurt in the past?
Maybe you are into casual sex and are none of the above things. But if the OTHER person is, you're in just as much trouble as they are. So go, have sex all you want, it is your choice; but please, do it with people you can trust. People who think there is nothing wrong with this choice don't seem to understand that in our world as it is today, casual sex with people you do not know and trust is very, very risky-- because you might get hurt, or you might hurt someone else.
That said, there is another problem from casual sex-- by making it an expected norm, we've labeled everyone who doesn't engage in it as either frigid or religious. The frigid/whore (or whipped/commitmentphobe) dichotomy is alive and well and it's perpetuated both by the "save it for marriage" types and the "sex please, and lots of it" types. Those of us who chose to only have sex in committed relationships, or to wait, often feel pressure from all sides to pick one or the other when in fact it is our body and we should be allowed to do what we want with it.
And almost everyone is guilty of it. If you've never heard of a party-girl type with many multiple partners and thought, "That's kind of... slutty..." then you've probably heard of some girl saying she'll wait for marriage and thought "She's just doing it because she's been brainwashed by her parents and her religion." Guys get it too-- they either sleep around a lot and are thought of as "afraid of commitment" or "misogynistic" or they never sleep around and they're "whipped" by some girl or a loser.
Sometimes I think humans MUST be designed to see the world in black and white. Nothing else explains why we are always so insistent that something can be one thing or another but not in between. Same as me or not like me but not in between. Democrat or Republican but not a mixture. Slutty or frigid but not in between. We actually have to fight that instinct. And we definitely should.