Leviticus 18:30

Therefore shall ye keep Mine ordinance, the precept covering this matter based upon natural law, that ye commit not any one of these abominable customs which were committed before you, and that ye defile not yourselves therein. I am the Lord, your God, and the just and holy God cannot endure such customs of abomination, as in those days, so in our times. There is a hint in this Chapter which has been sustained abundantly by history, namely, that any relaxation of sexual purity will usually be accompanied by the perversion or denial of some fundamental doctrine.