I always think of this passage from Zizek's The Parallax View. The iterations on sexuality provide an example of "dialectical progress"

The starting point here is copulation a tergo, the sexual act in its animal, presubjective immediacy; we then go on to its immediate (abstract) negation: masturbation, in which solo self-excitation is supplemented by fantasizing. (Jean Laplanche argued that masturbation-with-fantasy is the elementary, zero-level, form of the properly human drive as opposed to the animal instinct). What follows is the synthesis of the sexual act proper in a missionary position, in which face-to-face contact guarantees that full bodily contact (penetration) remains supplemented by fantasizing. This means that the "normal" human sexual act has the structure of double masturbation: each participant is masturbating with a real partner. However, the gap between the raw reality of copulation and its fantasmatic supplement can no longer be closed; all variations and displaces of sexual practices that follow are so many desperate attempts to restore the balance of the two.

Thus a dialectical "progress" thus first goes through a series of variations with regard to the relationship between face, sexual organs, and other bodily parts, and the modes of their respective uses; the organ remains the phallus, but the opening to be penetrated changes (anus, mouth). Then, in a kind of "negation of negation" not only does the object to be penetrated change, but the totality of the person who is the partner passes into its opposite (homosexuality). In a further development, the goal itself is no longer orgasm (fetishism). Fist-fucking introduces into this series an impossible synthesis of hand (the organ of instrumental activity, of work) and vagina (the organ of "spontaneous" passive generation). The fist (focus of purposeful work, the hand as the most tightly controlled and trained part of the body) replaces the phallus (the organ out of conscious control par excellance, since its erection comes and goes independently of our will), in a kind of correlate to somebody who approaches a state that should emerge "spontaneously" in a well-planned instrumental way (a poet who constructs his poems in a "rational" way, for instance, is a poetic fist-fucker). There are, of course, further variations here which call for their speculative deduction: in masculine masturbation, the vagina, the ultimate passive organ, is substituted by the hand, the ultimate active organ, which passivizes the phallus itself. Furthermore, when the phallus penetrates the anus, we obtain the correct insight into the speculative identity of excrementation and insemination, the highest and the lowest. There is no room here to explore further variations to be deduced: doing it with an animal, with a machine doll; doing it with many partners, sadism, masochism...the main point is that the very "progress" from one form to another is motivated by the structural imbalance of the sexual relationship, which condemns any sexual practice to eternal oscillation, between the "spontaneous" pathos of self obliteration and the logic of external ritual ("following the rules"). Thus the final outcome is that sexuality is the domain of "spurious infinity" whose logic, brought to an extreme, cannot but engender tasteless excesses like those of spermathon contests - how many men can a woman bring to orgasm in an hour, and so on....for a true philosopher, there are more interesting things in the world than sex.