The Bible says sex is a tie between a couple: it's the most deep kind of involvement a couple can have. It is not a physical expression only, but involves spiritual and emotional areas of our life. It means whoever isn't a virgin brings all the ex-partners he/she slept with to the new relationship because when they had sex, they shared a part of themselves, causing a bond that generates emotional and spiritual connections that can affect the new relationship (even though they might claim they are fine because they used "protection").
Sexual intercourse appears to be a threshold at which man and woman enter into a new relationship that is biochemically oriented toward staying together, geared toward their unity as a couple. (...) Chemicals released during intercourse, such as oxytocin and vasopressin, introduce a unique effect that promotes a series of pair-bonding behaviors. The primary exposure of the system to these chemicals seems to trigger a response that permanently alters body chemistry, and consequently behavior tendencies, making the individual (animal or human) more receptive toward his or her partner. Once the bonds are established, the disruption of these bonds causes great distress. (...) Such data seem to show that the biochemistry of the body associated with sexual union is optimally designed to be experienced within the context of an intimate and permanent relationship.
As you can see, the scientists are proving what the Bible has said many years ago, when God said about Adam and Eve: "That is why a man leaves his father and mother and is united to his wife, and they become one flesh" (Gn. 2.24). One flesh. Isn't that powerful? So I concluded that:
Because it's such a deep tie between a couple, God reserves it to the marriage, where both can express their love fully and shameless, through sex.
Every time you share your body with someone, you're using it for something that it wasn't made for. It's like using a potty for serving soup or using a Versace dress to clean the floor - it will work, but it's not what it was made for. And the action not only sounds unacceptable, but also dishonor the creation - the Versace or the soup - and in our case, our bodies and the SEX.
2b Each man should have sexual relations with his own wife, and each woman with her own husband. 3 The husband should fulfill his marital duty to his wife, and likewise the wife to her husband. 4 The wife does not have authority over her own body but yields it to her husband. In the same way, the husband does not have authority over his own body but yields it to his wife. 5 Do not deprive each other except perhaps by mutual consent and for a time, so that you may devote yourselves to prayer. Then come together again so that Satan will not tempt you because of your lack of self-control. 6 I say this as a concession, not as a command.