God made us to be sexual beings, and places us into families, which are created around a sexual relationship (between husband and wife). When that sexual relationship becomes disordered--sometimes through adultery, other times through a failure to be truly intimate (not just "having sex" for personal pleasure), or in the relationship becoming platonic--all of society is threatened.
Pastor David Petersen wrote a while back, "I'll let you in on a little secret: [sexual struggles/sins] are big, sweeping, nearly universal problems." He is right, and the longer I am a pastor, they seem to be some of the deepest problems as well, the most difficult to rectify.
I am struck by how Leviticus spends an entire chapter (18) addressing nothing but this. For long periods of history, Christianity has been interpreted to be prudish to the point of seeing all sex as evil or at best for those who are weak. The Reformation put forth a much more healthy view, but healthy marriages are in short supply, also among Lutheran clergy (who, given our theology, one would think would excel at loving their wives). What is really going on in Lev. 18 is an absolutely pro-family legislation, cultivating a sacredness around the relationship between a man and his wife. Nothing should intrude upon that. John Kleinig puts it this way in his excellent Leviticus commentary:
The prohibitions [of Lev. 18] are not primarily concerned with individual rights (as Westerners are today) but with the integrity and survival of the extended family, for it was threatened most radically by incest and disordered sexual relationships. (pp383f)