There is no such thing as sexual morality per se. Put less dramatically, there is no morality special to sex: no act is wrong simply because of its sexual nature. Sexual morality consists in moral considerations that are relevant elsewhere as well being applied to sexual activity or relations. This is because the proper concept of sexual activity is morally neutral. Sexual activity is that which fulfills sexual desire.  Sexual desire in its primary sense can be defined as desire for physical contact with another person’s body and for the pleasure that such contact brings. Masturbation or desire to view pornography are sexual activity and desire in a secondary sense, substitutes for normal sexual desire in its primary sense. Sex itself is not a moral category, although it places us in relations in which moral considerations apply. It gives us opportunity to do what is otherwise regarded as wrong: to harm, deceive, or manipulate others against their will. 

As other philosophers point out, pleasure is normally a byproduct of successfully doing things not aimed at pleasure directly, but this is not the case with sex. Sexual desire aims directly at the pleasure derived from physical contact. Desire for physical contact in other contexts, for example contact sports, is not sexual because it has other motives (winning, exhibiting dominance, etc.), but sexual desire in itself has no other motive. It is not a desire to reproduce or to express love or other emotions, although sexual activity, like other activities, can express various emotions including love.

Overly restrictive sexual ethics derive from definitions that wrongly build these extraneous motives into the concept of proper sex. Our definition’s being morally neutral in building in no end except physical contact implies that no conduct otherwise immoral should be excused because it is sexual, and, more important, that nothing in sex is immoral unless condemned by factors that are wrong-making elsewhere. Sex without love is condemned by those who think that proper sex must be an expression of love; sex without physical contact is simply not sex in its primary sense, not condemnable on that ground.

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"Overly restrictive sexual ethics derive from definitions that wrongly build extraneous motives into the concept of proper sex."

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What sexual conduct is then wrong on ordinary moral grounds? Primarily that forced on others without their rational consent. Rape is the most obvious example, involving not only physical assault, but humiliation, invasion of privacy, and most often lasting psychological harm, all of which are separately wrong-making in other contexts. Only slightly less obvious in this category is sex with minors, which might seem a counterexample to my main claim in being considered wrong because sexual. But again such activity is not only coercive, since children are incapable of rational consent in this context, but psychologically harmful in the long term, coercion and infliction of harm being paradigm wrong-making features of any actions. Finally, clearly in this category as well are sexual activities with subordinates: doctors with patients, lawyers with clients, bosses with employees, and teachers with students. Such relations are again exploitative and in a slightly less obvious way coercive. There are borderline cases in this category too, such as sex with a person under the influence of alcohol or drugs or with prostitutes, sometimes but not always wrong, depending on the plausibility of seeing coercion in the context.