They have the freedom in Christ to do so. Here Paul is referring to the longing, the desire, not just the physical desire, but the emotional longing for intimacy, for family, for all that goes with having a partner in life. For some singles, that kind of attitude can become a consuming fire, an idol in their lives. But the apostle's advice here is not at all unlike his word to married couples in verse 3. The simple command to act requires one to probe and learn in many areas of life previously ignored.
Some singles are unrealistic in the high standards which they demand of potential spouses.
Others have yet to learn how unfulfilling is the singleminded pursuit of career advancement and material success. Singles who take to heart the apostolic assignment therefore, and who want to be married, must ask themselves, and their brothers and sisters in Christ, what it is about them that needs changing so that they may indeed enter into the married state. If they determine to do this, all the energy which they formerly put into feeling miserable and unthankful for their circumstances can now be channeled toward the positive direction of what they themselves can do to change. Our assignment in life, whether married or single, is a gift from God. Singleness is a wonderful gift.