Except that, in this case (fornication), the mind isn't being tempted by Satan, but by chemical reactions within the body.
Sex is a biological process, not a psychological choice. One's sex drive is not triggered by the marriage ritual. The natural urge to procreate cannot be halted with Devine aid. It can be willfully muted to an extent, but the only way to remove it is, again, biologically, through (I believe irreversible) surgery.
Following your lead, I'm guessing that God knew exactly what he was doing, and he wouldn't design human beings expecting us to control an innate biological process using our minds. First of all, it cannot be done; we can no more control hormones using will-power than we can control infectious disease using the mind. Secondly, such a plan would be cruel. As God expects us to satiate hunger with food, he expects us to satisfy our libidos with sex.
We have two subjects at play here: Sex and marriage. There appears to be a conflict. In settling the conflict, I need only see that sex is unequivocally a product of God's creation, but the origin of marriage is debatable. As such, any unrealistic standards such as prohibitions against non-marital sex must be seen in the context of human culture, not God's plan.