Yet, despite the varied themes and wide range of mitzvot included in this Torah portion, Rashi's comment on its opening lines suggest a much narrower scope regarding what holiness is all about:
RASHI: You shall be holy: Separate yourselves from sexual immorality and from sin, for wherever one finds a barrier against sexual immorality, one finds holiness, [for example:], “[They (the kohanim) shall not take in marriage] a woman who is a prostitute or one who was profaned…I, the Lord, Who sanctifies you [am holy]” (Lev. 21:7-8); and, “he shall not profane his offspring…I am the Lord, Who sanctifies him” (Lev. 21:15); and, “They shall be holy…[They shall not take in marriage] a woman who is a prostitute or one who was profaned” (Lev. 21:6-7). - [Vayikra Rabbah 24:4-6; and see also Sefer Hazikkaron]
The end of last week's portion, Acharei, dealt almost exclusively with sexual sins. However, Kedoshim only discusses sexual sins until much much later, almost as a side note. We see that Rashi himself makes no reference to the previous portion, perhaps due to the very fact that the Torah does not to include these few verses as part of Acharei, but rather as the beginning of an entirely new parashah. (Incidentally, on non-leap years, the two portions are usually read together)
What is Rashi's reason for focusing in on the role of holiness vis-a-vis sexual sins? He is certainly not denying that all the other mitzvot mentioned lead a person to holiness. Rashi seems to be stating the following: "While it is true that all the other deeds depicted below lead to holiness, do not think that you can truly achieve it without separating yourself from sexual immorality and from sin."
Unfortunately, society today has completely distorted the role of sexuality. It is true that the Torah certainly does not see sexual relations as a sin, much less as the "original sin." Sex, within the proper parameters of a marriage, is a mitzvah, the very first mitzvah mentioned in the Torah ("be fruitful and multiply"). Yet, outside the context of marriage, sex is clearly prohibited, mainly because it lacks holiness. Because of various factors, ranging from Freud to marketing, society has greatly cheapened the concept of sexual relations in general, and the female body in particular, greatly cheapening ourselves in the process.
Again, the Torah does not advocate Victorian puritanism, burkas, or anything of the sort, yet we must understand that if we desire to have true holiness in our lives, we have to be in control of our sexual urges, and not the other way around, having them dictate our actions. This certainly applies to married couples, too, and applies not just to sex itself, but to everything we see, touch, hear, and (consciously) think about. (Perhaps that is why Rashi mentions "sexual immorality" and "sin" as two separate concepts. The Ramban explains that one can be a Naval b'Reshut haTorah - a disgusting person, acting in ways that are technically permitted by the Torah).
It might sound a little radical, but as soon as we are in control of our animal side, our soul is free to soar. It is incredibly liberating, and allows for a much stronger and heartfelt connection to G-d. The beauty of Judaism is that it advocates that once our spiritual side is in control and the Torah's precepts are followed, then pleasure is not a sin at all - it is holy, and is itself a praise of the Almighty. Rabbi Chanan Morrison writes the following in the name of Rav Kook:
What is a brachah? When we recite a blessing, we are expressing our awareness of God as the ultimate source for this pleasure. But there is an enjoyment greater than the sensory pleasure that comes from eating food. Eating entitles us to recite a blessing and thus connect with our Creator. We experience an inner joy when we realize that every form of physical pleasure was created with the opportunity to refine the spirit and uplift the soul. (http://ravkooktorah.org/KDOSHIM59.htm)
Above, Rabbi Morrison writes about eating, but, as mentioned in Rashi's comments above, it all comes back to sexuality. This concept is reflected in the writings of many other great sages, such as Rebbe Nachman of Breslov and Rebbe Shlomo of Radomsk, on the topic of Shmirat Brit Kodesh ("guarding the holy covenant").