There's a fundamental difference between between these two scenarios -
Let's take a math example: 1+1=2
even though we cant prove WHY 1+1 =2 we can, in almost every case see that the result of adding 1+1 is two. You have 1 car, you buy another car, you end up with 2 cars, you have one cat, you get another cat, you have 2 cats, etc. This has been shown to be accurate by personal example in every case I've ever seen (I can think of one example otherwise in chemistry, but that has another explanation)
Actually we
can prove that 1+1=2 from our axioms. It seems you are basing what you believe by what you think or "clearly see" to be true. Math has no such restrictions. You can assume some things (which we called axioms or postulates) to be true and build up a complex mathematical system from applying rules of logic. The things we call axioms are usually so obviously true and cannot be proven from other axioms (if they can be proven from other axioms then they are called theorems). Axioms are like "atoms" of the math world -- they are the basic building blocks.
For example, in a branch of mathematics called Euclidean Geometry, one of the axioms is something like "Given 2 infinite parallel lines, they will never intercept" (it's been a long time since I took this course so I might be a little off on specifics).
This seems so obviously true. Mathematicians were surprised that this statement could not be proven by the other 3 simpler axioms that made up Euclidean Geometry. With those 4 axioms you could prove every theorem that you encountered in your high school geometry course. Newtonian Physics was based on Euclidean Geometry. It seemed to model the real world.
However, since it's an axiom this means it cannot be proven true or false (even though it seems obviously true to us). Therefore, it's perfectly permissible to consider it to be false. If you consider that statement to be false then you enter the branch of mathematics known as Non-Euclidean Geometry. With the 3 axioms of Euclidean Geometry (not including the axiom that states that "2 infinite parallel lines will never intercept"

you can prove a bunch of theorems. In the Non-Euclidean Geometry world, parallel lines can intercept!
Guess which branch of mathematics is a more accurate model of the real physical world? It's Non-Euclidean geometry! The Theory of Relativity is based on Non-Euclidean geometry.
Anyway, my original point is that these axioms are unprovable. We accept them on faith (some of these axioms actually seem false -- 2 parallel lines intercepting at some point). This is no different than someone accepting God on faith.
Incidently, the logician and mathematician Godel once proved under any logic/mathematical system, we can always come up with statements that cannot be proven true or false. In other words, logic cannot answer all our questions. There will always be questions where logic will not be able to answer. When I first read about this, I realized how stupid Vulcans are to base a society entirely on logic.
However, lets say two people have sex, eventually, the woman gets pregnant, she has a son, who is supported by both his mother and his father (who still aren't officially married) Eventually, this child's parents are able to afford to send him to Harvard. He graduates from Harvard with honors and goes on to discover a cure for cancer. If these two people,who were never married, hadn't had sex, people would still be dying of cancer. I'd consider this a GOOD outcome from a situation involving premarital sex.
You see the distinction? If EVERY (or even the overwhelming majority of) premarital sexual incounter ended up like the first situation, it would be easier to see that premarital sex is bad, without needing proof, however, that's not the case.
It seems that you are missing the point that I (and others) were trying to make. Have you carefully read the posts from people like
isildur and others? I don't want to repeat everything but I thought their arguments were very clear.
We are claiming that pre-marital sex should be considered bad behavior (or immoral) because it promotes an environment where multiple sex partners are the likely outcome. We do not want a society where the average number of sex partners is too high (because the risk of disease, out-of-wedlock births etc increases with more sex partners).
To me the connection between pre-marital sex and multiple sex partners is very obvious. Let's say we have an official rule of behavior that says it is okay to have pre-marital sex as long as you love this person. Sounds reasonable. Let's say a guy has sex with his first love. How many cases do you know where the guy marries the first girl he has sex with? Very few from my experience. What does this imply? This guy probably has had sex with a few other women before he finally settled down. So this policy does not discourage having multiple sex partners.
However a policy of "waiting until you are married before you have sex" does discourage having multiple sex partners.
These moral codes should not be used to judge an individual. They are only general rules that we as a group should strive for.
Now I know a lot of you guys will say, "This is not a realistic rule! No one can live by those rules. We should replace it with something reasonable!" It's true, I don't expect a lot of people to actually be able to achieve these goals. But these rules are something we should
strive for. Just because it's not realistic to expect everyone to get an A, does that mean we should replace the rule of "go for an A" to "go for a C"?
I really think the real reason why a lot of guys want to dismiss many religious and moral codes is to ease your own guilt. However, I can think of two possible ways to ease your guilt:
1) Change the rules.
2) Keep the rules (because this is what all of us should strive for and it makes for a better society). Accept the fact that we are human and that most of us will not able to live up to these moral codes but not feel guilty. What we should do is just try our best to live by these rules.
When you think about it, isn't choice #2 really what Jesus is all about? From what I've heard, God knows we are human and unable to live by these rules. But that's okay. He sent his Son to die for our sins so we can all go to heaven if we accept his gift? Is that the correct interpretation?