Musonius Rufus: "On Sexual Indulgence"
Musonius advanced the distinctive view that the natural function of sexual activity is only to produce offspring; hence all other relations, heterosexual or homosexual, are improper and reveal a lack of self-control. Even in marriage, sex for pleasure alone is wrong. The goal of marriage is the rearing of many children (abortion and exposure of infants being contrary to nature) and the cultivation of companionship between spouses, which is both a a symbol of and the foundation for social relations generally. Though we may suspect the influence of Roman social values on this teaching, Musonius supports his position with arguments based on natural teleology. His central theme is the importance of self-control in the service of a rationally articulated understanding of human nature.
I borrowed a copy of Musonius Rufus "The Roman Socrates" by Cora Lutz (which seems to be the only English translation of Musonius) and have copied out Musonius' Discourse XII: "On Sexual Indulgence" (pp. 85-9, trans. from Gk. by Lutz):
Not the least significant part of the life of luxury and self-indulgence lies also in sexual excess; for example those who lead  such a life crave a variety of loves not only lawful but unlawful ones as well, not women alone but also men; sometimes they pursue one love and sometimes another, and not being satisfied with those which are available, pursue those which are rare and inaccessible, and invent shameful intimacies, all of which constitute a grave indictment of manhood. Men who are not wantons or immoral are bound to consider sexual intercourse justified only when it occurs in marriage and is indulged in for the purpose of begetting children, since that is lawful, but unjust and unlawful when it is mere pleasure-seeking, even in marriage. But of all sexual relations those involving adultery are most unlawful, and no more tolerable are those of men with men, because it is a monstrous thing and contrary to nature. But, furthermore, leaving out of consideration adultery, all intercourse with women which is without lawful character is shameful and is practiced from lack of self-restraint. So no one with any self-control would think of having relations with a courtesan or a free woman apart from marriage, no, nor even with his own maid-servant. The fact that those relationships are not lawful or seemly makes them a disgrace and a reproach to those seeking them; whence it is that no one dares to do any of these things openly, not even if he has all but lost the ability to blush, and those who are not completely degenerate dare to do these things only in hiding and in secret. And yet to attempt to cover up what one is doing is equivalent to a confession of guilt. "That's all very well," you say, "but unlike the adulterer who wrongs the husband of the woman he corrupts, the man who has relations with a courtesan or a woman who has no husband wrongs no one for he does not destroy anyone's hope of children." I continue to maintain that everyone who sins and does wrong, even if it affects none of the people about him, yet immediately reveals himself as a worse and a less honorable person; for the wrong-doer by the very fact of doing wrong is worse and less honorable. Not to mention the injustice of the thing, there must be sheer wantonness in anyone yielding to the temptation of shameful pleasure and like swine rejoicing in his own vileness. In this category belongs the man who has relations with his own slave-maid, a thing which some people consider quite without blame, since every master is held to have it in his power to use his slave as he wishes. In reply to this I have just one thing to say: if it seems neither shameful nor out of place for a master to have relations with his own slave, particularly if she happens to be unmarried, let him consider how he would like it if his wife had relations with a male slave. Would it not seem completely intolerable not only if the woman who  had a lawful husband had relations with a slave, but even if a woman without a husband should have? And yet surely one will not expect men to be less moral than women, nor less capable of disciplining their desires, thereby revealing the stronger in judgment inferior to the weaker, the rulers to the ruled. In fact, it behooves men to be much better if they expect to be superior to women, for surely if they appear to be less self-controlled they will also be baser characters. What need is there to say that it is an act of licentiousness and nothing less for a master to have relations with a slave? Everyone knows that.
Lutz, Cora E. (1947). Musonius Rufus "The Roman Socrates". New Haven: Yale UP.