Fascinatingly enough, both halves of this week's double portion of Acharei Mot-Kedoshim include elaborate lists of sexual prohibitions, from incest and adultery to homosexuality and bestiality. Clearly, the Bible is emphasizing that sexuality outside of the sanctity of marriage is problematic, to say the least. 
Any individual who lives his life for the main purpose of continuing to live is doomed to fail.
But what is problematic is the strange order of the Biblical passages. Chapter 18 of the Book of Leviticus (Acharei Mot) commands, "You shall keep my statutes.... And live by them" (18:5), which is followed by twenty-four verses of sexual prohibitions, and the warning that the penalty for promiscuous and licentious conduct will be no less than exile - a "vomiting out" - from the Land of Israel. Next to the last "sexual prohibition" is sacrificing one's child to Molech (Leviticus 18:21). Then comes Chapter 19 and the second Biblical portion of Kedoshim, which gives a long list of laws of "holiness", from revering one's parents, to guarding the Sabbath, to leaving behind portions of one's field for the poor, to refraining from oppressing the stranger, to maintaining honest weights. Chapter 20 then opens with the prohibition of sacrificing one's child to Molech (Lev. 20: 1-5), only to be followed once again by a long list of sexual prohibitions, concluding this second portion with the command, "And you shall be holy to me, because I the Lord am holy." (Leviticus 20:26)
Hence, after the description of the Yom Kippur sacrificial service, here is the order of the laws found in the two portions we read this Sabbath: the command to live by G-d's laws; the sexual prohibitions; the transgression of sacrificing to Molech; the main commandment and continuing commandments of holiness; the transgression of Molech; the sexual prohibitions; and a final charge of holiness. Why interrupt the sexual prohibitions - which are largely repetitive - with the portion of "holiness"? Why conclude the sexual prohibitions the first time they appear with the prohibition of Molech and begin the second list of sexual prohibitions with the prohibition of Molech once again? And, above all, why open the sexual prohibitions the first time they appear with the command to live by G-d's laws when sexual immorality - along with murder and idolatry - is a rare prohibition for which one must give up one's life rather than transgress?
Let us deal with our last question first. I have often written that any individual who lives his life for the main purpose of continuing to live is doomed to fail; after all, no one has ever left this world alive. Hence, the only meaningful life worth living is the life dedicated to an ideal more significant than any one human life - and such an ideal is the Divine concept of morality. Yes, live by my laws - but in order to live a meaningful life and eventually to become indelibly connected to eternity is to live your life within the backdrop of ideals more important than one individual life; i.e., the eternal ideals of the Kingship of G-d.
The Hebrew word kadosh is generally translated as "holy" and is the most frequently used "description" of G-d, the transcendent G-d, the "wholly other" G-d, the G-d who is above and beyond the limitations of nature and instinct. Our Bible commands us to also be holy; as much as possible we must strive to free ourselves from the seductions and blandishments of the physical, materialistic world, and even of the instinctive and natural sexual drives which can often lead to sexual immorality. Hence, it is quite fitting that the list of sexual prohibitions should lead into the Biblical portion of "holiness" and be even repeated once again within the rubric of holiness.
To serve G-d and to attempt to emulate Him means to strive for holiness, for the ability to say "no" to one's more 
Sexual morality is such a major test that the sexual prohibitions are repeated twice.
materialistic instincts; and the area of sexual seduction is the most difficult to control, overcome and eventually sublimate. Sexual morality is such a major test that the sexual prohibitions are repeated twice.
But where does sacrificing one's child to Molech come into this picture? There are many ways of destroying a child - and one of the most pernicious is when a child grows up in a home in which the faithfulness of the marital bed is compromised. There is no greater comfort for children than feeling the warmth and security of loving parents, and there is no greater psychological turmoil for children than seeing parents warring against each other, charging each other with infidelity and hypocritical conduct. One sacrifices one's child to a false god when one fails in one's most fundamental obligation to those whom we brought into the world; i.e., to provide for them a secure haven with parents whom they can trust.
Perhaps the verses in these two portions are the most significant - as well as the most difficult to maintain - of all the verses in the Bible.