There are many reasons to teach that sex is only to be practiced within the context of a monogamous marriage. God created sex to be a pleasure enjoyed within that protective boundary and to do otherwise is to defy Him and live with the innate consequences. These consequences include the obvious potential physical health problems, but I believe it goes much deeper than that. There is a trust between two people who have kept themselves for each other alone that is probably impossible between people who’ve not practiced that level of self control. For example, my sister-in-law never worried about my brother straying when he was deployed for over a year because she had already witnessed his self-restraint during their engagement. She saw the clear exercise of his integrity even when she hadn’t fully understood it early on. When one has only had a single partner, there are no performance comparisons to nag at you. You have the enjoyment of first discovery together without feeling the pressure to keep up with all the other partners. There’s just a whole lot less baggage.
Then there is the idea of imprinting. I believe the bonding with your one and only is just not there if there have been more. As a woman, I can only imagine the damage done to have given of yourself sexually to a man only to be cast aside for someone else. How many pieces of yourself can you give away without becoming very damaged and/or jaded. It’s really just a very bad idea.
We have taught our kids the importance of doing as we did, abstaining until marriage. We have not taught them sex is bad, but that it is wonderful when treated as something special and done within the parameters of God’s design. Guess it worked, as both our kids are waiting. They will have no worries about what they bring to the marriage bed.