These debates always make me question why people are so involved in the sex lives of others to be offended or off-put by what they get up to in the bedroom.
Personally, I don't do the casual sex thing. I feel uncomfortable with that level of intimacy with people I don't know that well, but then I don't really see any problem with people who do have casual sex. If you believe it is immoral to sleep with a vast number of people, then fair enough, don't sleep with a vast number of people, but to judge others for it seems strange. Unless you feel pain with every penetration of an unwed vagina, like a puritanical version of Harry Potter's burning scar, then I don't see how it personally effects you enough to care.
It is usually leveled at women over men, and that's probably because of societal reasons. The religious idea of women's worth being determined by their sexuality, and the idea of a woman either being pure or a whore that dates back to the days when a wife was something to be traded for two goats and a handful of olives. Whatever. People like sticking to old ideas. It's hard to shake off long held societal ideas.
There's the self respect argument, that people who have lots of sex don't respect themselves because if you were doing it, you would lose your self-respect, but if you think about it, the shame and guilt associated with sex is a societal construct. You don't get confetti and a banner straight from heaven if you have your first time on your wedding day, and the devil doesn't claim a kitten for every pair of teenagers having clumsy drunk sex on a pile of coats. If you don't want to do it, don't, and don't go throwing dirty words about for the ones that do. They just have a different moral code to you, and chances are the only people it could possibly hurt are consenting.
There's the STD argument, that people who are having casual sex are causing everyone to die horrible deaths from horrible diseases, and while STDs are a problem, they aren't if you're careful (unless you're particularly unlucky). You can sleep with two people in your lifetime and spread some horrible disease to lover #2, or you can sleep with two hundred and never contract a thing. Being unsafe is ill-judged, but if that's what they want to do, and both parties are comfortable with forgoing precautions, then that is their choice and they'll have to deal with the possible consequences. I don't see how a moral judgment can be made over that really. If you sleep with a girl and she gives you herpes, then you accuse her of being a 'slag' then surely you're as much of a 'slag' as she is for having unprotected sex with her?
Then there's the 'sleeping with another woman's man' argument, and I can see where morality comes in here, but so often you see women blaming other women for their boyfriends and husbands straying. The woman isn't the one with the responsibility, unless she's your best friend or something, the man is the one with the responsibility to you. For all you know, the woman doesn't know that she's with a married man, or she's told you're splitting up, or you're evil. The man is the one who knows the conditions of the relationship. Isn't blaming some stranger for the cruelty of your boyfriend a sign of low self-respect when you should be telling him where to go, and thanking the girl for bringing to your attention that he is an absolute bellend?
That's just my opinion. You can reverse the roles, where the man is the 'slag', or where both people involved are men, women, toasters, whatever, but the message stays the same. Let other people live their lives on their own terms when it's not hurting anybody, and live yours on yours.