Sex is an intimate interpersonal activity among humans and various species of animals that is used for procreation, pleasure, and expressing love for one another. Within Christianity it is considered a privilege and a blessing for heterosexual married couples, as it is meant to unite the man and woman together as "one flesh", and any other use for sex outside that, including homosexuality, is considered sexual immorality. As paraphrased from Paul the Apostle's first letter to the Corinthians in the Message translation:

There’s more to sex than mere skin on skin. Sex is as much spiritual mystery as physical fact. As written in Scripture, “The two become one.” Since we want to become spiritually one with the Master, we must not pursue the kind of sex that avoids commitment and intimacy, leaving us more lonely than ever—the kind of sex that can never “become one.” There is a sense in which sexual sins are different from all others. In sexual sin we violate the sacredness of our own bodies, these bodies that were made for God-given and God-modeled love, for “becoming one” with another. Or didn’t you realize that your body is a sacred place, the place of the Holy Spirit? Don’t you see that you can’t live however you please, squandering what God paid such a high price for? The physical part of you is not some piece of property belonging to the spiritual part of you. God owns the whole works. So let people see God in and through your body. (1 Corinthians 6:16-20)

In the world that existed during the Tribulation in the Left Behind series, sex was permitted to be engaged in by anyone without any specific limits, though those who became Christians in that period chose not to do so outside the boundary of marriage. Television networks that were owned by the Global Community were now free to show all forms of sensuality at all hours of the day. Carmela, who became a member of The Place, was a prostitute who used sex in order to survive and to get her hands on more drugs.

With this lack of restraint also came rape, which was predominant in the world before the Tribulation but now was accelerated to such levels that Global Community citizens sought to arm themselves for their own protection. As people who were against God, either militantly or passively, chose to follow their own desires instead of God's, they were subject to the various judgments that were unleashed upon the world, and ultimately sentenced to the Lake of Fire upon Jesus Christ's return.

During the Millennial Kingdom, sex was the primary if not exclusive way for the "natural" population to increase, being as the "glorified" had no capacity for reproduction and were asexual, having no need for sex since they were now immortal and have a relationship with God that fulfills all their desires. Members of The Other Light such as Rehema are known to have had children outside of marriage. At the end of the Millennium, all the remaining naturals that became believers also became glorifieds and went to live with God and Jesus Christ on the new earth that replaced the old earth, thus fulfilling Jesus' words in the Gospels that those who are worthy of the age of the resurrection "will neither marry nor be given in marriage, but will be like the angels in heaven."

At that point, it is not known if the non-asexual unsaved in the Lake of Fire are able to have sex, but they would presumably retain the desire to do so. It is reasonable to assume that they would not be able to reproduce.