The first commandment that G-d gave to Avraham was to live in the Land of Israel. What was the second commandment? To guard the holiness of his sexual life, as it says:
“This is My Brit that you shall safeguard, between Me and you, and your seed after you, every male among you shall be circumcised” (Bereshit, 17:8-10).
The meaning of safeguarding the Brit, as the Zohar explains, is not merely the obligation to circumcise our male children, but to guard our sexual holiness as well.
These two commandments were to be the foundation of the Jewish Nation, in the great Divine plan of establishing G-d’s Kingdom on earth, whereby all mankind would come to know and serve its Creator.
This Divine goal is to be achieved through the holy life of the Jewish Nation in the Land of Israel. Thus, G-d first brought Avraham to the Holy Land. Then, to build Avraham’s offspring into a Holy Nation, G-d gave Avraham the Covenant of the Brit.
The third phase of this Divine process transpired generations later, with the giving of the Torah, the holy constitution of the newborn Jewish Nation, so that the Jewish People could conduct both their private and national lives in the holy fashion that the Holy Land demands.
This NATIONAL sanctification of G-d, by the holy Jewish Nation, in the holy Land of Israel, is what brings G-d’s Presence into this world, bringing the nations to declare that the G-d of Israel is King over all the earth.
Thus we see what an important role the laws of modesty and proper marital relations play in building the holy Jewish Nation and establishing G-d’s Kingdom on earth! How important it is to review these laws again and again so that we all can do our share in furthering this great holy enterprise, and not, G-d forbid, hamper its actualization in our time. For, as the Zohar repeatedly teaches, no transgression does more damage in removing G-d’s Presence from the world than transgressions to the Brit of sexual holiness.
Therefore, adult readers are encouraged to review the detailed Laws of Marital Relations as explained by Rabbi Mordechai Eliahu; and the articles, the "Holiness of Marital Relations" by the Ramban, and “Keys to a Holy Union,” by the Kabbalist, Rabbi Eliahu Leon Levi, posted on the jewishsexuality.com website.