It's the, genetically speaking, jackpot of all rewards, no? Should come with the biggest natural reward the body can produce; should be even better than eating something tasty when you're about to starve to death. Well, I'm actually not sure about to what extent reproduction would be prioritized over survival, or even things like long-term group survival/status-seeking/etc, in humans. We know the end goal is reproduction (and/or/if not then genetic survival by other means), but always prioritizing mating isn't how it's set up. Still, shouldn't be that much different from jacking off to porn (for the first couple of times, not after desensitization sets in), especially if using realistic sex toys/dolls/vibrators/whatever. And, of course, some hard drugs (maybe even large amounts of alcohol?) should have sex beat. Also, meditation can really have some nice effects, I had one experience where I felt some love for the universe type thing, years ago now. There's a lot of variety depending on the actual sex. Having several fertile partners lined up should have the biggest effect (gender difference should exist between heterosexuals here, with women theoretically preferring a good quality mate over a high quantity of them), but I can't imagine it beating heroin or a well-orchestrated porn-binge (pre-desensitization). I haven't done hard drugs or had sex, though, not that such anecdotes would be terribly relevant.
Now, by observing sex-havers I get the impression that it's not a super big part of their life. It's just that, a part. Maybe a crucial part, but so is general bodily maintenance that we don't think is anything fancy, or going to work. They have their various goals, their friends, their food, whatever. Age will be a factor here. I read that the average sex lasts like 5-15 minutes or something. For the people that really drag it out, it's still (routinely) under an hour. So, that leaves 15 hours of waking time without sex for the people that really put the time in. Now, they could do it twice, maybe three times a day (unlikely). So, a three hour total (choosing the longest duration in combination with the higher frequency). Time wise, still a minority for the most sexually active (I'm assuming youths). Like with hunger and thirst, there's a base level for not feeling need, then you can go beyond that (good nutrition vs just surviving). They can have sex once, then not feel the need for some duration of time, then it's time for the normie to have sex again.
It's also way more opportunistic/psychological than hunger or thirst, in my experience. Which makes sense, if there are no partners around it doesn't pay to get aroused or feel like you need sex. Why we who never get the opportunity to have sex will have (hopefully) small pockets of frustration/need in combination with triggers/thoughts, rather than a constant feeling. Actually, I don't even think thirst of hunger would be constant, but when I've been hungry it's just way more present than horniness during nofap. Still, nofap is not even a good comparison (at least not for me), since I'm constantly in close proximity to my source of illusory sexual partners (computer). Also, might see women on screens in non-pornographic settings, and the brain can't tell that it's not a viable mate. Also, also, porn can be a powerful addiction, affecting the "natural" mental activity. In nature, there could actually be a complete lack of viable mating options (real or imagined), which should then divert the bioenergetic resources to second-hand genetic survival and survival as such. Masturbation would then only be done for maintenance, but you'd then still feel some pleasure (way less than with real sex). It's also entirely possible that not even imagination would not be used during such masturbation.
To answer the question; it depends on which rating, by which person, in reference to which sex by which person at which time, in comparison to what. Or, if we assume the average rating by the average person, the average sex and and the average substitute; the perception should be right around correct., for the simple reason that this is one of the areas where people will know what's what. At least if what's weighed is a subjective feeling of pleasure, as is how I interpreted the question. Now, we could compare sexual intercourse to completely other activities rooted in other goals (some religious goal, political, knowledge, health, suicide, money, etc), and decide if it's then overrated. In those cases the answer should obviously be yes most of the time, since you can't work on stuff whilst having sex. Well, it could be done, and people have multitasked in this way, no doubt. But the sex would still add to the amount of this you could do "at once" (switching between tasks quickly), and there would be a limit. If several people then help out, and/or technology is used, this limit could be moved, potentially very far. But then it wouldn't really be someone doing things while having sex, as much as it would be assisted delegation followed by sex. And, if the goals are achievable by not actually doing things, then we're back to square one since the goal isn't at all related to the choice of activity.
It is magical with the right person. It is not overrated but I do think it has been cheapened. It shouldn't be a sole focus in life but it definitely brings great joy and pleasant feelings, improves well being.
