The first narrative is that all people are just expected to pair off into monogamous relationships, gay or straight, short-term or long-term, legally-binding or not, but always with a clearly defined beginning and ending of a relationship, and that a permanent commitment (i.e. marriage) is the mark of a "successful" relationship. This is the narrative that the majority of people seem to live by, and the one most represented in books, movies and television. It derives from classic medieval romance stories, but with a few minor alterations for the modern age. While one-night-stands do exist in this narrative, they are always defined as "meaningless," and even regarded as "immature." In this narrative, sex is almost a formality, as if having sex with a person is a way of certifying that, yes, this is a romantic relationship, as opposed to just two friends hanging out together. Movies and television typically don't go into too much detail about how this type of sex is performed, as the focus is on the relationship aspect, but it is implied that there's a "magical" property to sex when it's with the right person, and that the act itself is relatively vanilla, with kinks and fetishes reserved for aforementioned "meaningless" sexual encounters. In short, casual sex exists, but the ultimate goal is always long-term monogamy.
All three narratives have a sort of "shadow" narrative, that follows the same overall belief as the main narrative, but with a dark additional element. In the case of the first narrative, the shadow is the stereotypical "nice guy" narrative, which implies that achieving the goal of long-term monogamy is a literal reward for having good character, with good character being defined mostly around adherence to the standards set by the narrative. That is to say, the more quickly and completely one commits to a relationship, the more deserving one is of having one. This can be used by some people to justify obsessive and even stalkerish behavior. The logic goes something like, "If obsession is a sign of devotion, and devotion is a good thing, then obsession must be a good thing, and should be reciprocated." This is obviously dangerous, because at best, it leads to arrogance and self-entitlement, and at worst, can lead to psychopathic behavior.
The second narrative is that sex is a cultural taboo, and taboos are made to be broken. This is kind of a narrative nested inside a narrative. The belief is that sex is good, but also the belief is that the belief is that sex is bad. This is the narrative in which it's "fun to be naughty." We see this in expressions like "bad boy" and "bad girl." This is also where we find the foundation of things like BDSM, roleplay, and of course, porn. Despite porn's obvious casual attitude to sex, there's almost always a paradoxical presumption that some external observer somewhere disapproves, hence all the "dirty little secrets" and "forbidden desires." It's based entirely around the concept that sex is fun, not in spite of, but because of the fact that it's breaking rules. Naturally, porn has very little to do with real life, but it does reflect a general attitude held by many people. Fortunately, most people respect that there are rules within rules. For example: sex with someone roleplaying as a minor is good, but sex with an actual minor is bad; watching a video about someone cheating on a partner is good, but actually cheating on a partner is bad; watching a video about or roleplaying as a boss using quid pro quo on an employee is good, but doing this in real life is bad; etc. This leads to a few problems. For one, a person might find sex boring unless there's some element of danger or of sinfulness about it, even if it's only pretend. For another, a person may become desensitized to fake risks and fake taboos, and start seeking out the real thing in the form of rape, incest, pedophilia, etc.
While the extreme rule-breakers are certainly a dark aspect to the second narrative, they don't really count as the "shadow" of the narrative, but instead, just a more extreme form of the narrative. The true shadow, is the person who does the complete opposite, by becoming prudish. These are the people who agree that sex is naughty, and that naughty is fun, but would rather be rule-followers than rule-breakers. For these people, religion can be a convenient excuse, but more often, it's a kind of indirect conformity. The person becomes sexually reserved or repressed because a sexually-repressive society was implied by someone else for some unrelated reason. It's similar to the well-observed effect that people tend to wear more drab colors in times of economic recession. People who feel powerless want to gain approval from "society" and from "the system," and so they subconsciously self-repress, to show that they are "good" people.
The third narrative is that sex is a status symbol for men, and a mark of shame for women. (This is the narrative in which LGBTQ+ becomes a complicating factor, and must be removed from consideration or labeled as "exempt" from following the script.) Simply, men seek to acquire as much sex as possible from as many women as possible. This allows them to report back to their peers, elevating their place in the social hierarchy. Men who lie to brag are considered the lowest of the low, but only when caught. Rapists, specifically those who attack women in dark alleys, are bad men, not because they violate autonomy, but because they are "cheating to win." Women are classified basically like video game levels, with some women being "too easy" and others "too impossible." Women should be just challenging enough, with more attractive women having a higher value, because of the opportunity to be more selective. This is the narrative that gives us all the classic baseball expressions, "scoring," "striking out," "grass on the field," "going all the way," "switch-hitter," "pitching," "catching," and of course, all the "bases." This narrative is a serious problem for women, because it bars them from enjoying sex without lowering their own value and status. People often point out what a problem this is for men as well, because sex becomes an obligation wrought with psychological pressure instead of an enjoyable activity, as well as this being a symptom of the whole "toxic masculinity" culture. Unlike the other two narratives, however, fewer people seem to actually subscribe to this thinking. Those that do seem to have it as a sort of default setting, presumed to be the narrative of society. To put it another way, if 100 men are in a room, and one of the men perceives women as trophies, while the other 99 disagree, the young observer may assume that the first man is typical of the entire room, while the rest are agreeing with him only in secret. Thus, a minority opinion can still be perceived as a majority, and in this way, create more peer pressure for sensitive individuals.
The shadow of the third narrative is the complete rebellion against the narrative, taking the form of sex-negative feminism or some form of misandry, either from women directed at men, or from men directed at themselves. This shadow narrative never quite took hold of the public consciousness the way the others have, but it does tend to crop up from time to time.
The one narrative that never seems to appear in the public consciousness is the idea of sex as a simple recreational activity, which is strange, because out of all of them, this is the most logical. The problem is that the first three are so deeply rooted in the collective psyche, that most of us can only reach this point after years of unlearning the previous three. All three narratives must be unlearned, as a person who accept the fault in one or two of them will likely default to the remaining one. For example, a person who views sex as a form of conquest (the third narrative) might later have an awakening, realize the error of his ways, and embrace monogamy (the first narrative). For another example, a person attempting to be monogamous (the first narrative) might later become bored or disillusioned with traditional relationships, and indulge in a kind of underground swinger lifestyle, fitting with the second narrative. Alternatively, a person following one of the main narratives, after an unpleasant experience of some sort, might fall into the shadow version of that particular narrative. Once in the shadow, breaking out requires unlearning the entire narrative. For an example of this, a person who has a sex-negative attitude, seeing it as "dirty," probably isn't going to embrace the second narrative, enjoying breaking rules. More likely, this person would have to erase the entire second narrative and embrace the first one or none at all. Only a person who has examined and rejected all three can truly embrace the fourth, fully sex-positive narrative, because all three are so unavoidable.
Even the fourth "alternative" worldview has a shadow, albeit not nearly as damaging. That shadow is the rare-but-vocal sexual liberation evangelist. This person may hold the belief that casual non-monogamous sex is not only something that should be socially acceptable, but should be applied universally. They may attempt to argue that a couple in a relationship is fundamentally wrong, having been brainwashed by society into going against their intrinsic non-monogamous nature. Granted, these sorts of attitudes are not exactly dangerous, they can be extremely irritating, because they fail to consider that the monogamous, the faux-transgressive, or even the celibate, may have found a form of happiness that works for them. To tell such people that their lifestyles are wrong or unnatural is no less disrespectful than trying to convince a person in a homosexual relationship to become exclusively heterosexual.
In conclusion, the sort of sexual or romantic lifestyle a person leads should be bespoke to the individual preferences, flexible enough to adapt to changing preferences, and never modeled after the expectations of perceived society. There is no "right," except what is right for the individual, and there is no "wrong," except that which does physical or psychological harm to oneself or others. These social narratives are simply fictions, not obligations or lifestyle blueprints.