If one plans to marry a person, why does God forbid having sex with that potential spouse before the marriage?
The first reason is that pre-marital sex sabotages married sex. Because pre-marital sex is forbidden in Scripture, it is always entered into in shame and secrecy and results in the arrival of dark guilt. Immoral couples don't run right out and joyously proclaim that they have just bit the dust. They hide it. Why? Their physical freedom is exchanged for internal psychological conflict. They become compromised in their faith by living against the verses that say "Do not commit fornication" (i.e. any sexual act outside of a committed marriage, witnessed by two witnesses or a public ceremony, first) and a loss of reputation.
Because pre-marital sex shows no restraint (i.e. moral self-discipline), it creates insecurity for the later marriage. Since no restraint was shown with their potential marriage partner before the vow, what is to keep one or both spouses from restraint with extramarital affairs afterwards? Sexual temptation doesn't just end with marriage vows. Adultery is rampant in our current culture. A failed lesson #1 is often followed by a failed lesson #2.
Pre-marital sex that results in conception will result in either murder (abortion) or shame at telling one's child someday that mommy and daddy didn't love the child enough to bring him or her into an uncompromised godly family. God's wishes/command in the matter wasn't important enough and the child himself wasn't important enough to display any discipline.
The underlying message to the maturing child is that Mom and Dad were selfish and indulgent...ruled by passion--not love for others--outside of themselves. They simply did not care about the hurt that they would impose upon their own child's chance at beginning life with a solid psyche, in a God-revering home. This sets a poor example for the adult chlld, years later.
A "spouse-to-be" may die in an unforeseen accident before marriage. It has been known to happen even the day before or on the actual day before the marriage ceremony. In this case the alive spouse who decides to get married again must tell that second potential spouse (if he/she has engaged in premarital sex) that he/she is not a virgin. This is no gift to the new spouse candidate who has saved himself/herself for marriage. It creates profound disappointment and it starts off the marriage in an atmosphere of mistrust.
Keeping the "letter of the law" is no answer, either. Heavy petting before marriage that stops short of the actual act creates a lurching sexual pattern/habit that may not be so easily broken after marriage. Sexual response is a continuum that was designed by God to be uninterrupted--it is meant to be fully complete every time. No "training wheels stages" are needed. When the full expression has been halted abruptly again and again just before its final destination stage, that can severely damage the body's own natural free consummate response in marriage later. Premarital sex is no gift to the marriage. Countless marital counselors of large numbers of disturbed marriages have found it to be an obstacle to what God designed to be joyously expressed at liberty within a committed context.
And if there have been multiple sexual partners, before settling down to the current engaged one, the behavior exposes the spouse-to-be to the possibility of sexually transmitted diseases, a shortened life, and misery. Because God made us, His well thought-out commands were designed to spare us from unnecessary disease.
There is no downside to saving oneself for marriage, the very way that God intended. Sex is a gift for a specific, narrow, fulfilling context.