I thank all the people who took time to comment about my question regarding casual sex. I think most of the contention and confusion in the comments arose from interpreting the hypothetical situation, which I an now sorry I brought up. One has to assume an average context to a hypothetical. If one starts dreaming up wild scenarios like, "Okay, say it's the last day of this guy's life..." then one defeats the purpose of the hypothetical. But still, there are too many variables to make that hypothetical question useful.
I have some uncertainty about casual sex, which is why I asked the question. My tentative answer is most like the fourth commenter, Anonymous, who brought up the virtue of pride. Promiscuity shows a lack of pride. Before the rise of the egalitarian New Left, discriminating was understood as a virtue. A discriminating man does not sleep with any slut who will say yes after 10 seconds of conversation.
To understand a lack of pride as immorality, one must get rid of every vestige of Christian or traditional morality, for they hold humility as a virtue. I think even some Objectivists struggle with the idea that lacking pride is immoral. Remember, morality is not primarily about what you do to other people, but about how you should deal with reality. Pride is a virtue because it means you strive to live as well as you can.
Sex between a man and a woman involves the woman submitting to the man. The man pursues the woman, wins the woman, conquers the woman, takes the woman and other verbs that make romantic love sound like a battlefield. The man penetrates and the woman is penetrated. If a woman submits too easily, then the victory is not as satisfying.
But I also sympathize with Tom Rowland's position. Casual sex might be inferior and not as satisfying as romantic love sex, and certainly a habit of casual sex -- promiscuity -- is wrong, but is occasional casual sex always wrong? I can't say that it is.
Don't tell me my position is like "If I only rob banks on Tuesday, then I'm still moral." Casual sex is not a crime. A better analogy would be, "I know that great art offers the more enriching, soul-satisfying experience, but sometimes I like to watch detective shows on TV." Or "I enjoy fine dining, but sometimes I only have time and money for McDonald's."
I think those of us who are not religious still have to watch for remnants of puritan hatred of sex in our thinking. Sex is a good thing. With a serious, committed romantic partner it is great; with anyone less serious it can still be pretty good.
Those are my thoughts. I am open to persuasion if I am wrong.