The term sexual aggression implies aggression somehow linked to the breeding process, though there may be displaced or dysfunctional expressions of this behavior, too.
From Male to Female
As odd as it may seem, male cats spring on a receptive female and bite her in the scruff, pinning her down as they intromit for a few seconds of orgasmic pleasure. The bite is not a faint one, either, and is performed with sufficient tenacity to allow the brief mating process to take place without the possibility of the female turning on the male or running away. The female's behavior both before and after mating may explain the male's forceful approach that effectively secures the target of his amorous attentions and at the same time protects him from reciprocal hostility.
From Female to Male
As a female comes into heat, her impending receptivity attracts one or more suitors who, typical males that they are, all want to get on with the business of mating in haste without much courtship and with all due haste. Not so, slow down, is the signal the female gives as she waits for the correct moment before lowering her drawbridge. Premature advances are met with aggression, hissing, spitting, and batting, until just the precise moment for optimum biological success. At this point she allows the advance, stoically accepts the neck bite, braces herself, and no doubt "thinks of England." However, the very moment the dastardly deed is done, and perhaps not least due to the sudden withdrawal of his nibs barbed penis, she lets out a cry, rolls over, and (seemingly) tries to take a hunk out of him, swatting and pawing as she hisses and spits various indignities. All in all, not the most amorous of encounters.
From Neutered Male to Willing Recipient
I have often heard owners describe the aggressive behavior of their neutered male cat toward their neutered female cat in in terms that sound, for all the world, like male mounting behavior. A recent case involved a small, blind female cat that was constantly attacked by her neutered male housemate. He would run up to her from behind , launch himself at her and bite her in the neck, pinning her for several seconds at a time. He engaged in this behavior so often that the hair was thinning on the nape of the female's neck and she was becoming extremely anxious about these intermittent and inescapable attacks. This was not territorial aggression, because the cats got on reasonably well at other times and did not appear to be related to fear or dominance. The only reasonable explanation was sexual aggression by the neutered male. That the behavior was suppressed by adding the odor of a male pheromone to the female's rump supported this explanation. Although sexual behavior by castrates may sound far-fetched, it is known that some neutered males continue to show male behaviors for some time after neutering, and "some time"can be almost indefinitely. This is because neutering does not make a male cat an "it," rather a neutered male (i.e. a male with no testes and thus extremely low blood levels of the male hormone, testosterone). Maleness is a property bestowed, in utero, by fetal testosterone as it first bathes the developing brain. The changes that result are permanent but are only fully activated by testosterone when it begins to flow during adolescence. When testosterone is removed (by castration), it is as if the lights are turned down, but not out. A dimmer switch analogy works well to describe this situation.
Sometimes, the recipient of residual maleness, mounting and aggression, is another male cat. I have even heard of it being directed toward objects.
It might seem odd that aggression and sex would be so intimately linked but , then again, they often are. Even the brain centers that control these behaviors are next door neighbors. That pain is involved in the mating process is no big surprise either, since pain stimulates the sympathetic nervous system, which in turn facilitates orgasm and ejaculation. The Beatles once sung, "You were told when you were young that pain would lead to pleasure." Apparently they were right. The entwining of all these behaviors and experiences in the procreative act is normal and natural phenomenon. But neutered cats that mount, bite, and otherwise terrorize other cats in the household should – for humanitarian reasons – have their jets cooled. So, if you see a red-blooded tomcat biting a female in the neck or a queen cat giving her suitor hell, you don't have to worry or assume the worst. However, if you see a neutered male beating up on an undercat, it's time to report this behavior to authorities who can do something to help address the problem.