There ain't nothing wrong with casual sex as long as BOTH know it's just casual sex. I am a humanist - I support the right for all humans to have the same rights. Feminism is a branch of humanism, so I am a feminist by default due to my being a humanist.
Real feminists support the right for a woman to live her life the way she wants to, provided it breaks no real laws. If a woman wants to have casual sex, so be it. That's her decision. And there are women who want casual sex. Probably not as many as men, but there are definitely some. If a man looking for casual sex meets a woman looking for casual sex, and they both know that it's just casual sex, then neither of them have committed a crime.
It's when men pretend to love a woman for casual sex, that problems arise. If you aren't willing to really invest in a relationship, you should be honest and say that. To fool a woman into having sex with you, by pretending to love her, is a jerk thing to do. Not something to put someone in jail over, but a pretty nasty thing to do regardless.
Now, I have to clarify something: I am, personally, a prude. I am as frigid a male as you can get. If I wasn't an atheist, I'd make an excellent priest. I do not, nor will I ever, engage in casual sex. But I don't judge others who do.
Let me ask you something: If two consenting adults have casual sex, what does it matter to you? How does it harm you? You can claim that it harms those involved, but there's no way to prove that. You can claim that it's a "sin", but even if you think it is, why not leave it to your God to settle? If they have contraceptives and protection, the risk of pregnancy is negligible. If they inform each other of any STDs, then again, there's no problem.
So while I wouldn't ever have casual sex (or, going by the way my life runs, and my basic personality, any type of sex, ever), I don't see why we should judge others who do. As long as sex is done by CONSENTING, MENTALLY SOUND ADULTS, what's the harm? Who are we to stand in the way? Sex with minors is evil, because they can't consent (they don't have the proper maturity to make those decisions). Sex with animals is evil, because, again, they can't consent. Sex with the mentally disabled can be evil (not always, but often is) because the mentally disabled person might not have the ability to understand what sex is, especially if they are severely disabled.
But consensual sex between two adults? I can't see any non-religious problem, provided that they are honest about any STDs they have.
I don't get the so-called Feminists who object to women having casual sex. Are you saying that you have the right to tell them what to do? That you know better than them? That is extremely insulting. You're basically saying "You're too stupid to know men are taking advantage of you!". I don't underestimate the intelligence of women. I'm not arrogant enough to proclaim that I alone know the way they should live, and many of these Sex-Negative Feminists are the preachy, in-your-face, I-Know-What's-Best-For-You types of moralizing jerks who are no less controlling than sexist men.
Feminism, as I understand it, is about giving women CHOICE. It's not about telling them how to live. It's not about trying to "show them the way!". It's about giving them the legal rights, the respect and the independence to live their life the way THEY WANT TO. Will that mean they make mistakes? Yes! That's part of life! We all make mistakes! You can argue that a life filled with casual sex is a mistake, that it will harm them later on. I disagree, but you can make that claim. However, at the very least, you should give them the right to make that "Mistake".
If you are a feminist, or a humanist, or anyone who wants equal rights and respect for all, you should at least be able to respect the intelligence of those who choose to live their life differently than how you would, and not be so arrogant to assume that you need to "enlighten" them on the "Proper" way to live, as if you have any right to do so.
If a woman wants to have casual, consensual sex with a man who also wants casual, consensual sex - SO BE IT! I don't care! And you shouldn't either! And if you think consensual sex makes your God cry, then let God take care of it.