Parshas Acharei Mos begins with the detailed description of the korbanos offered on Yom Kippur and concludes with the list of prohibited relationships that are categorized as gilui arayos. Although there doesn't appear to be any connection between these two sets of halachos, the krias haTorah on Yom Kippur seems to indicate that they are in fact related. We read the beginning of Acharei Mos on Yom Kippur morning, and at Mincha we read the end of the parsha, seemingly continuing where we left off earlier in the day. Is it just coincidental that the two krias come from the same parsha, or is there a real connection between the beginning of Parshas Acharei Mos and its conclusion?
The two principles that are at the root of both the avodas Yom HaKippurim as well as the prohibitions of giliu arayos are kedusha (holiness) and tahara (purity). The avoda is performed in the Kodesh HaKadashim, the holiest place in the world, by the Kohein Gadol, the holiest member of the Jewish People, who attains the highest possible level of kedusha. Because this kedusha cannot be reached in the state of tumah (impurity), the Kohein Gadol first brings a korban to atone for any sin involving defilement of the Beis Hamikdash through impurity (Vayikra 16:16). The two stated goals of sprinkling the blood of this atoning korban on the mizbeach, "to purify it and to make it holy", articulate the goals of the entire avoda.
Holiness and purity are also the basis for the prohibitions of giliu arayos. The mitzvah of "Kedoshim tihiyu - you should be holy" (Vayikra 19:2) is understood by Rashi as referring specifically to giliu arayos. Furthermore, the term "tumah" appears six times in the closing seven pesukim of Acharei Mos which give a final charge to not defile ourselves and Eretz Yisroel through giliu arayos.
The terms holiness and purity are the basis for the relationship of a permissible marriage, the antithesis of giliu arayos. Marriage itself is referred to as kiddushin. Jewish marriage is governed by the halachos of taharas hamishpacha which preserve the purity of this holy relationship. The ultimate breach of this purity - the sin of the woman who is a sotah - is referred to as an act of tuma (Bamidbar 5:13).
The source of all holiness in the world is Hashem. Parshas Kedoshim begins with this theme - "Be holy because I am holy" (Vayikra 19:2) - and ends with "you should be holy for me because I, Hashem, am holy" (Vayikra 20:26). Hashem's role as the exclusive source of kedusha is also manifest in the halachic distinctions made between tashmishei kedusha and tashmishei mitzvah. Unlike tashmishei mitzvah (objects used for mitzvos but lack kedusha, such as lulav and tzitzis), Tashmishei kedusha (items endowed with holiness - such as sifrei Torah, tefillin, and mezuzos) cannot be discarded but rather must be buried. What distinguishes the objects that are elevated to the status of tashmishei kedusha? All tashmishei kedusha have the name of Hashem written in them, and thus are holy because Hashem's name is holy. Tashmishei mitzva do not have Hashem's name and as such have no inherent sanctity. Further, the ultimate object of holiness is the Beis Hamikdash which has within it the presence of Hashem, not merely His name.
As stated earlier, tumah must be removed to enable kedusha to be present. Tahara must therefore be preserved in the Beis Hamikdash, when eating korbanos, etc. Similarly, the kedusha of the relationship between husband and wife must also be maintained through tahara. Giliu arayos and non-observance of taharas hamishpacha are the antitheses of that kedusha. Just as the kedusha and required tahara of the Beis Hamikdash emanate from the presence of Hashem, so too does the kedusha of marriage stem from Hashem. Where do we find Hashem in the context of a marriage? Chazal in Meseches Kiddushim observe that the mitzvah of honoring ones parents is comparable to honoring Hashem, as there are three partners in the creation of man - Hashem, a father and a mother. Hashem's presence thus manifests itself through marriage. For the kedusha of Hashem's presence to be part of a marriage, proper tahara must be maintained.
Kedusha and tahara of both the Beis Hamikdash and the relationship between a man and a woman cannot be taken lightly. There is a special mitzvah of shmiras hamikdash - guarding the Beis Hamikdash to prevent its defilement. We cannot risk impurity which would contradict kedusha; we are required to safeguard that which is holy. The shmiras hamikdash is reminiscent of the fences placed around Har Sinai lest the holiness manifest by Hashem's presence there be defiled. Similarly, the kedusha of Hashem's presence in marriage cannot be taken lightly. Before listing the specific arayos the Torah commands us "do not come close to violating gilui arayos (Vayikra 18:6)." The Rambam interprets this to refer to the Torah prohibition of any physical contact between a man and a woman who is prohibited to him. We thus see that the Torah itself enacts a fence around the prohibition of giliu arayos. Following the prohibitions of giliu arayos, the Torah commands us to "safeguard the charges of Hashem" (Vayikra 18:30). Chazal interpret this as referring to the obligation to enact rabbinic prohibitions distancing us from actual giliu arayos.
Hashem's presence must be guarded by us with the utmost care. Kedushas Hamikdash and the kedusha of marriage must be maintained by adhering to the highest standards of tahara. Proper precautions in these realms will enable us to be blessed by Hashem's presence in the Beis Hamikdash and in our homes.