What is wrong with pre-marital sex? If your spouse finds out that you are bad at sex after you are married, won't your spouse leave you for someone else?
Here lies a classic argument of "Let us do evil that good may come" (Romans 3:8). You want to prevent your future spouse from walking out on you and committing adultery; so you justify committing fornication. As the old cliche says, two wrongs don't make a right.
I would like for you to consider to articles that address this question.
Sexual intercourse is an act that is learned, just as walking and talking are learned actions. If your body's organs are functioning properly, intercourse can take place. How well you are able to perform the sexual act depends on the commitment and desires of the married couple. The fact that a couple engages in the act of sex prior to marriage does not impact their ability (or lack of ability) to engage in the same act after marriage. Nor does it determine their ability to improve over time. The justification for pre-marital sex is not an argument, but a grasping for something to relieve the guilt of wrong-doing.
Implied in the argument is the assumption that marriage only exists for sex. Such could not be further from the truth. Marriage is primarily for companionship. If marriage were solely for sexual relationships then those who are medically unable to have sexual intercourse would not be able to marry -- a limitation that is not found in the Scriptures. For more details concerning what marriage is truely about, see "What is Marriage?" from the study Preparation for a Lifetime.