Why hold that the genuine goodness of sexuality is natural but not chosen without a sense of moral obligation?
The natural good of sexuality is the natural purpose of sexuality. The natural purpose of sexuality is coitus for procreation and the life long monogamous bonding of males and females. If this natural purpose were a premoral good, people naturally choose it independent of any thought of what they are morally obliged to choose. In reality, though, people do not choose the natural purpose indendently of moral rules.
In a previous post, I stated a fundamental moral principle of male sexuality: a man should engage in coitus only in a lifelong marital commitment. I admitted that justification of this principle requires difficult empirical work. Theoretically, the empirical work would result in a description of life in accordance with the principle which would naturally lead people to choose living in accordance with the principle. This empirical work can never be completed satisfactorily. A description of a moral life may be appealing. But not appealing enough to make it a natural cause of choices.
The empirical evidence for the good of sexuality does not function as empirical evidence usually functions. Usually, empirical evidence guides us to agree that something is the case. For instance, we might bring a die into better light to see that it is really blue rather than black. If we had never thought about the matter, someone might point out that a die, from a pair of dice, is a cube and that not only is it the case that the cube has six surfaces. Counting the surfaces leads us to see that a cube must have six surfaces. This is a situation in which empirical considerations provides a proof. Similarly, factual considerations about sexuality in marriage are intended to lead us to appreciate that the sexuality which is in accordance with moral rules must be desirable along with being desirable. In reality no empirical descriptions are compelling enough to be a proof about sexual moral rules.
Each person must be led to appreciate that there is a good of sexuality by a moral awareness of laws requiring pursuit of this good. Significant appreciation of the good of sexuality is attained by pursuing it under constraints of these moral laws. Or maybe, as Christine Emba shows a better teacher is regret from seeking sexual satisfaction in violation of these moral laws. In sexuality the moral laws teach us the good the laws promote and protect.
There are no special people, philosopher kings, who come to see beyond the slightest shadow of doubt the goodness of the good of sexuality. Perhaps the only goodness of sexuality we can all be brought to see is the goodness of being bound by rules.
I hesitate, though, to propose duty as a good we pursue. Duty has no attraction by itself. Duty is not selected for its own sake. It is selected as the alternative to the chaos of choice by whims of our unbridled inclinations.