The Preacher says: See, this alone I found, that God made human beings straightforward, but they have devised many schemes (Eccl 7.29).
Sometimes I wonder whether or not this is most evident in the case of sexual ethics. My own position is more or less the natural law position of the Roman Catholic Church: the natural goal of the sexual act is procreation, so that engaging in it in such a manner as precludes this is immoral. This excludes the use of birth control, masturbation, gay sex, etc. It's a very cut-and-dried position, straightforward and candid, grounded in what is supposed to be an obvious observation of the function of the genital organs. It's supposed to be an example of how God made us very straightforward.
If you deny this natural teleological view, and things quickly get grey. Is masturbation immoral? Is watching pornography immoral? Is non-standard sex, such as sodomy, immoral? Is gay sex immoral? Is pre-marital sex immoral? Is consensual, safe incest immoral? These are questions to which there are no obvious answers in the literature, at least among those who reject the natural law position.