2) If you gave in to your impulse to have premarital sex, you'd feel religious guilt over breaking an arbitrary rule.
Yes.
3) Feeling guilt is not good if it's over something arbitrary, therefore, we must assume that it's not an arbitrary rule.
Agreed. So the problem here is whether or not the prohibition is arbitrary.
I'm not saying you're lying. I have no doubt you really tell yourself it's not arbitrary and that it's a good practical rule. It's just that, well, you couldn't prove it was (like you could with, say, "no killing people").
I certainly have evidence that says it's a good practical rule. For one: marriage is good for society. Pre-marital sex can delay one's desire to marry, or even discard it permanently. For two: Christians believe that marriage is a sacred committment. And that anything that could potentially harm that union is harmful. Competing with your spouse's other lovers could certainly do that. Third: I don't recall if you have a LDS or Christian background or not. Nonetheless, Biblical language is replete with sexual references. There is a reason why we are called "the bride of Christ". The sex act is holy, wonderful, and a gift. to regard this gift as a mere plaything is to be condescending to God. There's more, but you get the drift.
Nothing quite screams "arbitrary rule" like the supernaturally loaded word "sin."
Fair point. Let's just call it not measuring up to this standard.
Yes, I feel guilt over making real mistakes (I don't call them "sins"; that a BS word) which is to say, if I actually harm someone else, I feel guilt about it. Rules concerned with the prevention of harm to others are not, in any way, arbitrary. Rules concerning consenting adults's non-harmful sex habits certainly are.
Then you do have a standard. YOu just don't think that pre-marital sex ever harms anyone.