In a very literal sense, this is not true. Discussions of the ethics of sex go back to Cynicism, (with Diogenes the Cynic masturbating in public on purpose) and forward to at least Schopenhauer and to psychoanalytic branches like Lacan. (Though still, much of this is about what is normal, and dispelling unconsidered condemnation, rather than about actual ethical reasons for actual sexual acts.) Critical Theory incorporates forms of feminism that involve strong attempts at analysing the ethics of sex.(Detour to advertise Starhawk and other "informal Critical Theorists", who make a definite point of not excluding sex as motivation, test case or general theoretical source material for her political/ethical positions.)
But in every other way, it is overbearingly obvious. Philosophy in the West after the Classical Period was hemmed in tightly by attempts not to contradict Christianity, and we still face direct refusals of students to even read the material on the basis of religious orthodoxy. So, much of this material gets very narrow circulation and little critical attention. (And therefore, much of it is absolutely awful, or hard to plough through as it comes couched in impenetrable framings for sheer self-defense of the author.) (I will not go back and edit that for overly-allusive language... I just.will.not...)
And sex is still something over which our society is deeply divided. Older concessions to the animal nature of sex, and the traditional disparity that places on the genders attracts more attention as soon as the subject arises in a critical forum than basic ethical treatment of sex itself as a real source of issues. You stop discussing sex and begin discussing gender- or oppression-based politics.
Further, as a general failing of ethics, we are too often looking at two extreme ends of a continuum of generality. Those at one end are obsessed with general principles, which will, of course, apply to sex as well as to anything else, which lets them ignore it without seeming to. And those at the other end cannot place judgement unless they are willing to go to a level of detail that most of us would find embarrassing to read.
Since Critical Theory, which tries to include both Freud and Feminism, aw well as Marx, this is slowly improving, as one cannot really 'feel' the problems of feminism or the male equivalent (which is usually some anti-hierarchical pacifism) without thinking through the crazy reduction of sex and violence to economic terms.
To pick a given framing that is not afraid of sex, I would point you at Starhawk's Truth or Dare, which is excessively political, but has that politics based in a pro-bisexual pacifist psychoanalytic reorientation of Marxist-feminism, and, despite that, can still be read. (Sorry to be repetitive, but she is really worth it.)
