The world's view
Out there in the world, sex is regarded as an activity between any two consenting adults.
In reality the world suspects there is something more to sex than that because even the world can see the consequences of sex (mis)used in certain ways. For example, HIV/AIDS and venereal diseases exist only because monogamy is disregarded. For example, gay relationships engender great health risks and medical costs, but of course it is politically incorrect to speak that truth. For example, costs to the state vastly increase when a marriage is torn apart by adultery. For example, the great misery caused by unfaithfulness in a marriage.
But in spite of these 'minor inconveniences' the world continues to act as though sex were just a recreational activity between any consenting adults. (The 'adults' bit is only at the moment: mark my words, the day will come when the western world will include in the list of acceptable possibilities what it now self-righteously condemns; sex with children: one fallen barrier makes every other barrier easier to fall. Soon the west will be like the Romans. It is only a matter of time).
Tragically these views of sex are now entering 'the church'. So we find that people who live together are welcomed into 'churches' without having to make any permanent promises. We find now practicing homosexuals accepted. Tragically some Baptist 'churches' are "discussing" gay relationships.
Scripture makes it clear that God's design is that sex is designed as his holy glue to keep together one man and one woman in marriage. Yes, since we will die, replacing ourselves is another function of sex. But its greater purpose, more than hinted at by the Song of Songs (which doesn't mention procreation), is to unite one man and one woman in love.
When Paul heard that some Corinthian Christian men were still visiting prostitutes he warned them saying:
"Do you not know that he who unites himself to a prostitute is one with her in body? For it is said that the two will become on flesh?" (1 Cor 6:16)
They thought sex was "just sex", but Paul says that sex is a powerful, mysterious, wonderful, mystical and even metaphysical glue which bonds a man and a woman together.
Inside of marriage sex is God's amazing gift. Outside of marriage is is the most destructive force under the sun. The river bed for this torrent is marriage. When allowed to flow outside that river bed, its floods destroy everyone it touches.
That's why Paul goes on to say:
"Flee from sexual immorality. All other sins a man commits are outside his body, but he who sins sexually sins against his own body." (18)
Sex is for marriage between one man and one woman, alone.