Jesus repeated and re-emphasized God's original plan and instructions. 19: 3-12 Jesus reminded the Jews of God's original plan. Uncovering the nakedness of another person is to view, touch (whether they are clothed or not) or expose the parts of the body that is considered one’s nakedness.
Marriage was for a man and woman to come together in a lifetime relationship where they would experience physical union exclusively with each other. The apostles condemned “sexual immorality." "Sexual immorality" in the N. refers to "sexual" intimacy outside of marriage (God's original purpose for it). In ancient times and even today these three areas on a woman’s body and two areas on a man’s body are obvious.
It is also the path to physical and sexual intimacy. Romantic feelings should be based on a solid foundation of love and concern for each other.
It should never drive the relationship, but complement it.
In marriage, romantic/emotional and physical intimacy has several purposes: (1) to express love and pleasure between a husband and wife (Prov.-19, Song of Songs), (2) to become emotionally and physically united as husband and wife (Gen. Gen. indicates that Adam and Eve were married and were naked and felt no shame (no disgrace or humiliation).
The reason that they felt no shame was because they were "innocent." They had no sin in them, nor was their sin in the world.
Also, this kind of situation can stimulate sexual desires either directly or by building romantic intimacy before marriage.
This is clearly shown (as mentioned earlier) in Song of Songs (see 4:9-10).
I'm assuming that we would all understand that living together and having a sexual relationship would be sexual immorality.
Living together (with sex) is the same as saying "I don't love you enough to commit myself to you and marry you, but I love you enough to live with you and have sex with you." That is not love. What about if we don't feel guilty about some romantic or sexual activity we're doing?
Jesus also condemned adultery and sexual immorality in the mind and desires as well as in action (Matt.-28, Matt.). This condemnation occurs in many verses some which are Heb. Any physical contact meant to stimulate someone sexually is forbidden.
Jesus pointed out the sin of the woman at the well for "having a man" (living together - sexual relations) that was not her husband. Sexual intimacy/contact is more than just the act of sexual intercourse.