Historically, without reliable methods of disease prevention and birth control we could not expect that our sexual behavior would not have very serious consequences for our personal well being and the integrity of our local relationships. But now that we do have the ability to have consequence free sex ( or sex without as much risk as it once had) how can we justify the reflexive disdain some of us have for members of the community who engage in many sexual encounters with many different partners? Is there a modern ethic of erotic desire that supports sexual restraint?
I can't speak to any justification of "reflexive disdain" for promiscuity (which seems a bit harsh to me), but I can certainly think of some basic economic/game-theoretic arguments one might make to prefer monogamous partnering, and to prefer non-promiscuous people over promiscuous people, at least for the purposes of being possible romantic/sexual partners. Analysis of sexual partnering is a huge topic which has been subject to a substantial academic literature in economics (see e.g., Grossbard 1978, Bergstrom 1994, Bergstrom 1996, Bauseister and Vohs 2004, Gould et al 2008, Lagerlof 2010, de la Croix and Mariani 2015). There have also been many popular articles on sexual economics (see e.g., here, here).
The following offers basic arguments for why a person might prefer a non-promiscuous partner to a promiscuous one. These arguments are by no means exhaustive, but they are all arguments I think are plausible, and could reasonably apply even in a situation where the risk of STDs is low/absent. For a broader view I recommend reading some literature on the economics of sex, partnering and marriage. This literature analyses incentives for the parties in the sexual marketplace and the resultant trends in decision-making.
Price signals, compatibility signals, etc.: A promiscuous person is (virtually by definition) one who is generally less choosy in their selection of sexual partners than average. In economic terms, this means that they set a lower "price" on the provision of sex to another person than most people. Hence, when a promiscuous person shows sexual interest in you, this is less of a compliment and less of a signal of compatibility, than when a non-promiscuous person shows sexual interest in you. This means that a person might rationally prefer to attract the attentions of a non-promiscuous person, since this constitutes a stronger signal of romantic/sexual compatibility than the attentions of a promiscuous person.
Put in less technical terms, if a man shows sexual interest in a woman, but he has a reputation for sleeping with every woman he meets, then she might rationally infer that he has no great personal interest in her, and would simply consider her a sexual conquest. On the other hand, if another man shows sexual interest in a woman, but he has a reputation of being very "choosy" and only having slept with a few women, then she might rationally infer that he regards her as quite "special" (i.e., quite compatible with him).
Inference of higher likelihood of adultery: General promiscuity while not in a monogamous relationship does not logically imply that a person is more likely to "cheat" on a partner while in a monogamous relationship, but these might plausibly be empirically related. Promiscuity involves a lower economic "price" for having sex, and this could plausibly lower the barriers to adultery, and thus make adultery more probable. A person might therefore rationally infer that a partner who is generally promiscuous (while not in a relationship) is more likely to "stray" while in a relationship. If monogamy and sexual fidelity are goods, and promiscuity increases the likelihood of sexual infidelity then promiscuous people will end up paying a "risk premium" in their attempts to form relationships - i.e., ceteris paribus they will be less preferred to non-promiscuous people.
Sexual intercourse as an act of intimacy: Sexual intercourse is considered by many people to be more than just a recreational act; it is also an act of romantic bonding and intimacy. To the extent that this is true, the value of the bonding act is diluted if sexual intercourse is spread more diffusely to many partners. Hence, to the extent that people enjoy sex for the purposes of romantic intimacy, they will tend to prefer exclusivity and will therefore tend to prefer non-promiscuous partners.
Perceived sexual prowess as a good: Most people like for their sexual partner to think that they are sexually proficient, and consider this to be a good. But a person's judgement of your sexual prowess will naturally be affected not only by your own abilities, but also by comparison of other sexual partners that person has had. The more sexual partners a person has had, the higher the likelihood that they have been with someone with greater sexual prowess. Hence, a person might rationally prefer a partner who has had less prior sexual partners, so that their relative sexual prowess is not "outranked" by many, if any, others.
Note that this consideration will tend to affect men more than women, since men are the "doers" who are expected to "perform" in sex, whereas women are more passive and have less expected of them. Hence, one would expect that men would value non-promiscuous women more than women value non-promiscuous men. That seems to me to be empirically true.
The above arguments are just a teaser; you might disagree with one or all of them. Nevertheless, these are the kinds of arguments you will generally find in economic analysis of human sexuality. To the extent that there are incentives to rationally prefer non-promiscuous people as sexual/romantic partners, this could plausibly give rise to social norms that give overly promiscuous people lower status in the sexual marketplace. As I said above, "reflexive disdain" for promiscuity seems a bit over the top to me, but there are certainly some reasons why promiscuity could emerge from sexual interaction as a negative social norm. (Final note: There are probably also some economic incentives that work the other way, giving promiscuous people an advantage in the sexual marketplace. The net effects appear historically to have favoured the non-promiscuous, in terms of the resultant social norms. Since you have asked for arguments that work against promiscuity, that is what I am giving you.)
I recently came across a journal article which linked moral offense towards promiscuity to the prevention of STDs. It's an interesting read, and you can draw your own conclusions. However, it points to a major limit to your argument: moral disgust at such things is a highly emotional thing. The disgust at promiscuity is not going to be toppled by a logical argument.
As such, we have to consider the timescales things operate on. We're no longer in logic's "once you're proven wrong, you're instantly and forevermore wrong." Emotional changes take time, especially over an entire society. The eradication of many STDs is quite new, in social terms. We've seen societies respond on this sort of timeline, such as its response to computers, but you do have to admit that the preference for promiscuity of a few is not going to have a response similar to the complete change in society computers have done.
In addition, STDs are not eradicated. We've cured some, and decreased the risk of others, but there are some that are still at large. AIDS would be a prime example. So even if the moral disgust against promiscuity is based on disease avoidance, which you and the article I linked suggest, there's still pressure on society from STDs. By your own logic, there's still a reason to spread such moral disgust.
All of that is purely from the perspective that the moral disgust against promiscuity is simply an evolutionary response to STDs. It does not even consider the position of those who have a different opinion about morals. Many view morals as something which comes from religion, and the majority of religious individuals believe in a religion whose primary writings come from before the modern advancements in science regarding STDs. Thus it is reasonable to expect those who believe their morals come from a religion will apply even more resistence to change than the response to disease does.