For understanding, you need to discern what is:
The moral natural order of female body,
The moral natural order of male body;
Thus, the moral natural order of the union of bodies (marital act, the fact of making one flesh between wife and husband, with sexual cooperation (mutual and reciprocal) in the human fashion.
For that, the theology of body ( dual humanity, sexed, sexual, alterity and complementarity), the theology of body of male (the theology of male orgasm), the theology of body of female (theology of female orgasms), the theology of the marital act. The theology is the work on the holy books, holy tradition, the holy anthroplogy that was explained by God. Yes, there is a theology of sex.
For that, there is the philosophy of body ( dual humanity, sexed, sexual, alterity and complementarity), the philosophy of body of male (the philosophy of male orgasm), the philosophy of of body of female (philosophy of female orgasms), the philosophy of the marital act: only the objective and realistic philosophy is in link with the natural law. Yes, there is a philosophy of sex. We have to study the actions, the facts in order to understand the primary causes, the Ends, the functions, the roles. Philosophy of the natural law (methaphysics, natural theology (theodicy), the epistemology for understand the natural order of the creation).
The marital act is a phenomenon (before, during and after the union of bodies). At each step, there are natural rules: mutual and reciprocal preparation with corollaries (the preliminaries non sexual, the sexual preliminaries in the human fashion, not like the other animal), the union of bodies with corollaries, and then after this union.
Each member of couple has to do his job, has to give his body, has to participate in the respect of his own nature, has to receive the body of the other.
To give, to receive and to offer in the respect of moral natural law of human beings.