I'm a bit partial to both sides of the argument.
It's true that guys and girls can rarely be friends without there being sexual tension between them. Having said that, if a female friend of mine approached me and asked for sex, I would likely say yes, but that's really only because of the huge influence my penis takes on my thought process. Assuming I'd have a clear head when met with this offer, I would likely decline. Why, you ask?
Quagmirian said:
Sleeping around is considered by some, and partially me, to be a sign of moral weakness.
This is why. While sex is often intended only to be a source of good (though not always clean) fun, our bodies are really designed to feel it as something more. If you have sex with a close friends, nine out of ten times, you're going to develop deeper feelings for them. After all, there's nothing more personal you can share with a mate than sex. It's very, very, VERY difficult to keep the "just friends" attitude with a girl after you've just plowed her bean field. And, no matter how much some may deny it, this is ALWAYS the case.
That's why casual sex is a sign of moral weakness. Deeper feelings of affection and intimacy almost always follow a session of good sex (with someone you already know well, obviously. I'm not saying you'll fall in love with that hooker around the corner). Sex is personal, and everyone knows that. People who are willing to have sex with a female friend and plan to treat it as just so, as opposed to something more, are in denial, because it's nearly impossible to wear the same face around her after the bed business is done.
So, in essence, casual sex is like lying to yourself. It's trying to ignore the deeper feelings that are typically associated with sexual activity. That in mind, no, I wouldn't likely have sex with a friend, unless I were planning for it to be the impetus of a relationship.