In short - If the players are truly open to sexual violence in the game then they could be presented with as many opportunities to engage in them as is fun for everybody involved. If its repetitive or tiring for some players then toning back might not hurt the overall enjoyment.
Also to be considered is whether they want to play the role receiving sexual violence (World invasion of the inescapable tentacle monsters) or the dealers of sexual violence (You have been defeated - time to get shafted). Or perhaps they wish to deal in rescuing others from sexual violence but don't want to be vulnerable themselves (Don't look disappointed. I'm here to rescue you!).
In terms of narrative: To target the question - sexual assault can be used to add to the drama of a situation. Witnessing a person being raped in an alleyway can be a very powerful motivator for a PC to get involved - and helps to shift the sympathies of a PC between two or more factions (rape seeming a quite evil act).
Sexual violence may also be portrayed as an act of devotion amongst the faithful to some particularly carnal (probably evil) god. It might be an act that followers actively engage in as normal - and the intensity of the description of the sexual violence might be portrayed to drive players to ponder the base animalistic desires of humanity. Perhaps it is a consentual society which is not actually evil but neutral - a quirky presentation of taboo as a cultural plus.
Perhaps it is something as fundamental as two persons in a sexually violent relationship that is also consentual - either partner can end it at any time.
It may even be possible to use the carnal inclination within a player to challenge a tendancy to shy away from watching or listening to such narratives - it basically becomes sexuality versus violence in the mind - and the blend might be possible to narrate in a way that steps the line between comfort and discomfort.
In terms of force of narration - being descriptive without over-elaboration will keep the game moving without losing the interest of the players. Describing the suspense in the air as the servant brings comes down the paddle upon the thief's bare body, leaving him gasping and whining as the dame of the manor looks on nonchallantly is yet another example of sexuality versus violence.
Sexual violence could be portrayed in many other ways but the below section are some fragmented possibilities not particularly aimed at narration:
Captivity/ dungeon scenes - just because the party member might or might not have useful information does not mean that the captors might not still choose to have a little fun with them - whether for reasons of gratification or humour. Goblins are a quintessential choice but other creatures work too - humans included.
A dryad might take a liking to a particularly charismatic party member and seek to lure him or her to its enclave - treating the character well and filling with promises while quietly conditioning the character into submission - Thorny growths might be used to try to keep the object of desire in, and would-be rescuers out.
The same might be true (in their own ways) of harpies, a vampire, succubi, or even the daughter of an evil lord (think Flash Gordon).
These are both circumstances where a complicated romance might arise.
There is nothing against adding humour or romance to the mix to soften the blow of a sexually violent encounter. Perhaps after being knocked out most of the character(s) find themselves in caged cell while one "lucky" character finds him or herself to be play room of a drow interrogator with clear kinky tendancies as s/he whips a large scarred rat-like creature that scurries within a steel wheel cage sending electric arcs dangerously close to the player's perch as the drows ebony fave breaks into an ivory smile as s/he sees your eyes fall on a series of spiked and long tools on the table to his or her side.
Various things could take the edge off this. Perhaps mention that the drow has a strange long-haired chubby spider in one hand whose fur stands on end as the static courses through the air. And references quirky lines from movies. Maybe mutters to himself like a mad scientist who has lost the plot - or maybe has conversations with the spider pet and the self just as often as the mirror.
Or perhaps the interrogator has an assistant who is completely enamoured. That could lead to a conflict of interest.
On a completely different note perhaps the 'working lads and ladies' in your world often sidle up to the characters with flirty comments and invitations for "a good time". A few might even 'reveal' their assets - and you never know when a girl might be a dude or vice versa - or might be hiding other secrets in the medical history - physical and mental.
Well... that was a little on how sexuality might manifest... not all of it sexual violence. This first part was included since this seems to be what the question is all about - how to approach sexual violence.
The following involve aspects that may be off-topic as per clarification but remain here as they may be employed by a GM to retain control for how inevitable, dark and/or explicit the implementation of sexual violence is in a given game:
There are three ways that come to mind.
Firstly there is threat management - threat being representative of the 'threat of witnessing or experiencing sexual violence' in this context.
It is possible to deliberately throw clues for the player to catch up on that certain choices made at a given juncture might lead to such a situation.
The way that this could take form in the way of plot clues - perhaps the main threat of sexual violence is from a particular clan of orcs or members of a particularly sadistic cult that has the unfortunate habit of marking their victims in a very visible way (this might provide a disincentive for players "hoping" to get their characters into such a situation - a little difficult to hide the brand of the cult order (or slave number) upon the arm, even harder if on the neck of the character.
The final threat indicator should be quite palpable - you can tone this down if the players don't appreciate the warning however. Once they have fallen down that rabbit hole - you should consider playing it out realistically - but not necessarily explicitely.
Upping the Ante with Side-Plots
In a way this involves the same mechanic that is practiced by GMs everywhere when managing the danger level of a campaign - and presumes that some places are just a lot more dangerous or high risk than others.
Sending a charismatic elf cleric alone to gather information about how Goblin pimps might be running a racket in the down-town slum areas of a city might just end up with that cleric having his or her horizons expanded in ways never experienced before
Threats can arise and fall as gangs and cults kidnap the innocent while barbaric clans pillage and ravage small villages. Choosing to face such opponents is also to face a raised risk of encountering sexual violence in some form or another.
However the world in general might not itself be terribly dark and dangerous a place - although bandits, lone psychos, harpies and carnal ghouls can always be pesky in certain geographic climates.
Time Lapse/ Fade to grey
In the event that a character has gotten into a situation that the player is clearly uncomfortable with then there is the option of greatly reducing the embarrasment through lapsing in and out of the details - or alternatively fading to grey.
Example of lapse - "The orc chief grins and advances. What happens next seems a blur in your mind and the treatment is brutal" (take a fortitude/ will save) As the minutes stretch into hours his mocking voice cuts into your mind every bit as sharply as the pain." (it is possible to add more lapses - in this case its just the one)
Example of fade-to-grey "The orc chief grins and advances. Next thing you know you are waking up in darkness. You feel truly exhausted and your spirits are low." (Test Constitution and Wisdom) (Test for intelligence also possible if the player seems keen on remembering important "details").