Excerpts from an article called "Biblical Sexual Morality"
From www.KosherTorah.com by Ariel Bar Tzadok.
Sexual "purity" plays a major role in the Torah concept of holiness. Indeed, the following parasha Kedoshim opens with the words, "You shall be holy, for I HaShem you G-d am holy" (Lev 19:1). Rashi's commentary interprets the close proximity of the commandments forbidding Arayot and the commandment to be holy to refer to one another. In order to be holy, the Torah states, one must first refrain from forbidden sexual practices.
One does not need to be a rocket scientist or Rabbi to realize that sexual temptations are often times over whelming. Indeed, according to the Talmud there is even a secret set of teachings surrounding the spiritual meaning of Arayot. Violations of these forbidden sexual unions are considered such a grievous sin that the consequences of violation may be universal and not just personal.
According to the Torah, the act of sexual intercourse is much more than a mere coupling of two human animal bodies. Just as human beings are composite entities consisting of both body and soul, so too does the act of sex contain both a physical and spiritual element. Indeed, throughout the Bible the act of sexual union is referred to as "knowing" one's partner. This act of knowing expresses an intimate connection of the ultimate level wherein which one's deeper most essence is revealed and shared.
Arayot on the other hand is described in another way. Arayot (forbidden sexual unions) are referred to as, "Galuei Irvah" (an uncovering of nakedness). Here, there is no knowing, there is only uncovering. The implication is that these forbidden expressions of sexuality are types of nakedness that need to be covered, i.e., concealed.
Due to the dual physical-spiritual nature of the sexual act, all sexual activity has spiritual consequences. It is because of these spiritual consequences that there exists a category of forbidden sexual unions. Due to the dual potential inherent in sexuality, HaShem ordained these Laws for the sake of safeguarding our souls. Proper sexual conduct enables us to touch the Divine. Forbidden sexual contact disconnects the violator from any connection to HaShem.