I think the whole discussion has gone on long enough that people are getting confused about the issue at hand. First, the disclaimer : No offense is intended and I believe people here are cool enough take this in the way I meant it.
"Why is *premarital* sex considered *immoral*?"
1)The discussion pertains to premarital sex. Not casual sex, not heathen sex, not "banging a syphillis-laden skank" (excuse my french) sex. In other words, the original question was "why is sex before marriage considered wrong/immoral?"
For those that have gone on about the dangers of STD's and the logical conclusion thereof that premarital sex is logically wrong, your argument looks like this :
Pre-suppositions:
i) There are many sexually transmitted diseases (STDs) being passed around.
ii) Having (premarital) sex puts you at risk of contracting STD.
Conclusion:
iii) Therefore, premarital sex is considered logically and, reasonably, wrong.
Laid out that way, the flaws in the argument become apparent. Firstly, the conclusion can only hold if the assumptions hold. If, someday, a cure is found to treat and prevent all STDs, then the above-mentioned few who argued this way would be very happy campers indeed. The analogy can be drawn to people who break in and rob stores during riots. The issue of that argument is therefore not premarital sex, but how to avoid getting STDs.
Secondly, extending the argument, does this mean that all acts that potentially endanger your well-being are considered inherently logically wrong and therefore immoral? That being the case, any act of self-sacrifice would, likewise, be regarded in the same light. I wonder what a certain Jew, pinned up on two sticks and dying for things he didn't do, would have to say about that.
Thirdly, the topic begs the quesion of morality. Since the Christian faith is, arbitrarily, just a system of morality, asking a Christian to argue morals without quoting religious dogma is just plain mean

At least invite em to justify their system of morals, then that'd be more fun
In addition, the discussion in general seems to have the assumption that the whole premarital sex deal is rotten for the female partner. If your man leaves you feeling like a tree that's been pissed on by a dog, there's definately a problem there. I wouldn't mind working that out with you, drop me a PM ;P .
A lot of people in this discussion have also mentioned the aspect of trust and commitment in a relationship. This is a side-issue to the main topic, but I have something to say about that anyway (it's late and I can't sleep

). The role of love-making in the building of a relationship seems to have been discounted. Hands-up those of you who have felt closer to someone after having sex. C'mon, hands right up.
There was a post about cultural beliefs and ideas, and how culture considers it taboo. Take a step back from good ol' USA and see that, contrary to popular belief, there ARE other places besides America. A certain culture in South America exists (I don't have the name offhand, could check it up if you like) in which the role of the father is played by the wife's brother, and the biological father is more like a playmate. Yet other cultures exist where homosexuality is considered the norm, because semen is considered evil and the men must keep it to themselves. Sound wierd to you? Heard "gangsta rap" lately?
People who argued that "you shouldn't try before you buy" are taking a leap of faith. If you are ready to do that with someone you are promising to spend the rest of your life with, I'm not sure if you're taking that promise very seriously.
Mention was made of a child coming into the world "on the wrong foot." Well, that child is coming into the world as a result of an act of love or intimacy (since we're not talking about rape here) and is WAY better off than a test-tube baby. At least the biological parents got steps 1-3 right (if you don't know what they are you shouldn't be reading this). Its also hard to prove that children conceived prior to wedlock turn out to be troubled people. Suggesting that the words "I do" have a magical and life-transforming effect on the fetus is rather far-fetched, to say the least.
Would someone please explain the whole "becoming part of someone" and "not sharing your whole being with another someone" after "becoming one with someone?" Sounds really messy, and last I checked, we've evolved way past amoeba by now.
Regarding the post about "our minds are so weak and feeble," I would like to invite the person who posted that message to take his argument further and bring it to its final, absurd conclusion. I use the term absurd in the philosophical sense. If we are not perfect, can never be perfect and never will be, and are too weak and feeble to comprehend things as they are, and never can, why should we even bother? We'll never succeed anyway. We'll never be able to progress from wearing a loincloth and scratching at fleas in the dark to wearing boxers and typing at keyboards in the dark. Even if we do manage the little triumphs, so what? 10000 years down the road, who's going to care about what I did anyway?
Well I care, for one, and I care because IT FEELS GOOD TO BE ALIVE.
Nothing personal, but if you're going to assume that you're too dumb or stupid to understand anything and never will, don't also make the mistake of assuming that others are like you. I respect your religion, respect my self-respect and those of others.
So, does premarital sex harm any one in any way? Assuming both partners do not have diagnoses on the DSM-IV, it is safe to say that the act itself does not. Free from the trappings of STD, it would then seem that there isn't anything wrong with premarital sex.
Btw, cheers to the man who raises his daughter up to not have to live with bad sex. Couldn't agree more. Cheers also to aihyah, whom I'm beginning to suspect is someone I know in person. (just from the things she's said.)
amdskip's hinting at a celestial "hotdeal" forum, where different people qualify for different rewards, just makes me sad. Marriage doesn't give you sexual affirmation. If it does, consider help. Marriage is a mere formality declaring to the rest of the people around you, and to the respective legislative entity, what you already know in your heart; You can't wait to share the rest of your life with the wonderful person in front of you. If you need help seeing that, then no amount of philosophical debate is going to help you.