Let us start with the cynical take: love is a fantasy.
But, that is exactly why the romance in Romancing the Stone works. The premise itself is Joan Wilder's life echoing her romance novels, fantastic adventure, treasure hunting, a mysterious stranger, relentless villains. If we buy into the premise, it is very easy to buy into the romance, because the one fantasy is nested quite comfortably within the other.
Damon Suede, possibly a pseudonym, writing for Romance University, says:
Romance authors love Romancing the Stone because it simultaneously ribs and respects the genre. [Screenwriter Diane] Thomas got romance at a visceral level, and for all the winking send ups of romantic cliché, she embraces them shamelessly... no small feat. It has remains a touchstone in Hollywood and within the Romance community for a simple reason:
Romancing the Stone remains one of the only blockbuster hits in pure romance.
Suede goes on to explain how classic romance films are generally melodramas, tragedies, or screwball comedies, and offers examples. Then, Suede gets eventually to
all the stuff that romance does so well:
-  showing transformation through relationships and personal growth
-  exploring internal discovery as opposed to external conflict
-  using intimacy to elicit overwhelming emotion
-  indulging in luxury, eroticism, and escapist fantasy
-  celebrating intimacy and sexuality in non-objectifying ways
...is the stuff that Hollywood really struggles to do at ALL, let alone simultaneously.
Regarding #1 on Suede's list, structurally there is a reason that Joan and Jack work as a couple as well as adventuring partners. Early in the film, Joan is a loner. She's friendly--she helps Mrs Irwin up the stairs, she barely resists the guy selling stuffed animals on the sidewalk, she spoils her cat as she spoils herself--but she seems to live a lonely life, spending all her energy on fantasy because it is easier than living her own life. If we don't get this from what we see of her apartment, we also get her restaurant conversation with her editor Gloria, in which Gloria is actively shopping the men in the place for potential suitors while Joan is having none of it. It is also Gloria who says outright that Joan is unprepared for Colombia.
By leaving for Colombia, though, Joan has made a huge first step in altering her life. Also, her sister's tragedy works as an excuse to adventure. Alexandra Sokoloff, Screenwriting Tricks blogger, calls Elaine's phone call "a classic and blatant CALL TO ADVENTURE".
There she meets Jack. He happened his way into Colombia while working a coffee boat. He loved the ocean--which suggests that he is as hopelessly romantic as Joan--but got stuck on land. Selling birds was, he says, "a fast way" to get what he wanted--his own boat--and "a hell of a lot healthier than dealing in" drugs. He's gruff, he's dirty, but he has no interest in selling drugs because he's a nice guy. But, he's also stuck. Maybe he'll sell this load of birds, make the estimated $15,000 off this load. But--and I don't know how much boats like the one in his photo went for in 1984--is that enough? Has he gotten mired in the quest rather than the goal? I mean, that coffee boat left him in Colombia a year and a half ago. His "fast way" is taking a while. Or maybe he lives in the fantasy just as much as Joan does. Getting it seems impossible so he remains in Colombia, living on the wrong side of the law and helping the occasional stranded woman.
He is living his own fantasy, but he also is Joan's fantasy. He even arrives in the film in the same way Jesse did at the end of the novel sequence that begins the film, silhouetted atop a hill.
Then, they're together. His fantasy has just been torn apart--his birds loosed by the bus accident, so he plays in Joan's. At first, it's for the money. (And her haggling is a great moment for her.) He's got a great line that sums him up pretty well: "Now, I ain't cheap, but I can be had."
She smiles when he lifts her suitcase, first to drop it by her for her to carry, and again when he picks it up to throw it into the jungle. She's still living the fantasy, seeing chivalry where there is none... Except, there is just a little. He throws away the suitcase to be helpful to them both; it is slowing them down. Just like he cuts the heels off her shoes. His need to get things done faster is actually practical. But, gradually he is also opening up to her as he forces her to open up to the reality of her circumstances.
Jump ahead a bit. Sokoloff has a great description for Joan after the high speed chase in Juan's Little Mule; she writes:
As Juan, Jack and Joan take a breather in a mountain field, Joan picks flowers, looking very sexily disheveled and glowing from their wild ride (more and more like Angelina). Or perhaps that's just because we're seeing her from Jack's POV, and he's seeing her in a new light after having read some of her steamy novel.
Also, he just saw her win over a town full of men with guns, he saw her fame (and her prowess as a writer) buy them a brief respite in Juan's house and then a ride to where they need to go to find a phone. Joan's old reality is bleeding into the fantasy even as the fantasy is bleeding into her reality. And, Jack is benefiting from it. One can also imagine that him opening up in the downed plane before, even if only because he was breathing all that marijuana smoke from the fire and drinking tequila, was maybe the first time he's really talked about his fantasy with anyone. Something about this woman invading what has become his turf has lifted him up out of his own mire. Her fantasy has a more immediate end. Helping her achieve it makes fantasy itself more feasible.
Coming back to Suede's list, what a film like this does for #2 is combine the internal discovery and the external conflict. This is something film specializes in, despite Suede's including it on a list of things Hollywood struggles to do... Or maybe Hollywood's problem is it focuses too much on the external so, without the right director, the right performer, the right writer, the internal gets lost, or comes across as artificial. Similarly, with #3, Hollywood gets stuck on intimacy, i.e. sex and often fails to convey the overwhelming emotion that leads to it. Romancing the Stone offers up something more like intimacy in the conversation in the downed plane. This is also when Joan finally asks Jack's name. He becomes something more than a fantasy to her in this scene.
The intimacy of their fireside conversation is echoed with the dancing later and the presumably post-coital nakedness in bed, which manages #5, "celebrating intimacy and sexuality in non-obectifying ways"; their nakedness is not salacious. Their conversation in that moment is just as intimate as their naked bodies pressed together. He's inviting her into his fantasy. Whether Joan (or the audience) can fully trust him is still debatable, but this moment is vital to the romance--Jack has already been injected into Joan's fantasy, but now she has become a part of his; they will sailed the world together. Earlier his fantasy was specifically about sailing away by himself. Now she is there, too. The film is still telling us not to completely trust Jack--he asked Juan and he asked the desk clerk at the hotel about a photo copier because he wants to copy Joan's map--but this inclusion of Joan in his fantasy is more telling in the other direction. Jack may still be living a fantasy, hoping the treasure at the end of that map can buy him that boat--and, as it turns out, it can--but his fantasy is changing. This is romance in a nutshell. This is love, at least in terms of fiction, in terms of Hollywood. A shared fantasy. #4 on Suede's list includes luxury (which this plot aims for but avoids in context), eroticism, which is what you get in the dancing and the partial nude scene instead of lustful sexuality, and escapist fantasy.
My cynical take to start this was that love itself was a fantasy. Romancing the Stone relies on that fantasy, on this idea that we might have, even if only fueled by other productions out of Hollywood, that some other person will complete us and our lives will be richer for it. But, it also relies on the idea that we need more than that for our lives to be rich. Angelina has been the main character of several of Joan's novels, yet she and Jesse are not tangibly a couple; she calls him her beloved, but her arrives as an afterthought, after she has already killed Grogan and saved herself. Like Jack does at the end of the film; he comes to rescue her but she has already managed the hard part. The fantasy is not for a knight in shining armor to come save her and take care of her, but for something more equal. Angeline can take care of herself but still wants Jesse. Joan, by the end of the film, can take care of herself, but still wants Jack. Love is externalized, which makes this, in my opinion, a much healthier view of romance than many another film offers. Love is just one fantasy. It is not the fantasy.