I recall a story
The one you're thinking of is "Displaced Person", a novelette by Ian Stewart in Analog, May 1987, set on the same planet as the sequel "Captives of the Slavestone" which was suggested in James Forde's answer.
where portals — as pairs of linked windows — were naturally evolved and used by native flora and fauna of the planet!
"I fail to comprehend why you wish to apply mathematics to a plant."
Marco rose to the bait. "Elza, you can apply mathematics to anything. Of course," he added in a rash of honesty, "you don't necessarily get a sensible answer, but that's another matter. However," he continued doggedly, "if you can tell me a better way to understand a planet of vegetable matter-transmitters, go ahead."
Elzabet looked at him scornfully down her aristocratic nose.
"The syntei influence everything on this planet," Marco went on. "For millions of years they've controlled every aspect of evolution. Some vegetable genius stumbled upon the idea of separate buds that remained connected to Mama by a wormhole in space . . . and knocked the stuffing out of the competition. A syntelic plant could thrive in a desert with just one component on moist ground. By shedding damaged parts it became almost indestructible. Then . . . the animals got in on the act. They learned to use the wormholes as shortcuts."
It was a ubiquitous natural resource, and the story explained the resulting society that had access to that, from the point of view of outsiders who were stranded there.
"So Qishite technology followed a line of development that you won't find anywhere else. No long-distance mechanical transport. Roads mostly short and bridges small, for local transport where it's a waste of time to break things down to the size of a large synte aperture. Few machines. Those that do exist are rudimentary and most make use of syntei. Heavy emphasis on botany, breeding plants for special tasks. Breeding for size—the stability problem places an upper limit. Treatment methods for dead syntwood to preserve the wormhole property. Endless syntelic gadgets. Falasyntei for communication, with a drum-like skin across the opening." Marco, having got into his stride, was now in full flight. "Wasyntei that let you fish from a barrel in your own garden. Kolosyntei instead of holovision. Drop a stone through a gravity gradient and you've got a gun. A rock amounts to heavy artillery. Whole economic structures build up. Control an intercontinental wyzand and you can name your price. Which, incidentally, means that transportation on Qish is no more free than anywhere else in the galaxy. Political systems evolve around economics. And then there's the elevation problem—you can go a thousand miles if you stay at the same height, easier than you can go twenty yards up or down. Slopes acquire enormous strategic importance.
"And more personal possibilities. You can scratch your own back, touch your right elbow with your right hand, chew chew the back of your knees. Social conventions can run the gamut from rigid prohibition to unlimited freedom. The sexual permutations are limited only by your imagination—"
More details: the outsiders were a man and a woman.
There are six outsiders, but they are separated, and this story is about two of them, Marco Bianchi and the Lady Elzabet.
There were B&W illustrations.
Three two-page black-and-white illustrations by William R. Warren, Jr. on pp. 144–145, 160–161, and 166–167.
Early on, he mistakes her “dress” for a net bag and tries to carry coconuts in it.
"I," she declared, striking a pose, "am an exotic dancer. You," she continued, poking him in the chest, "are an assistant to the ossivore trainer. And I might add," she stated haughtily, "that it took all my powers of persuasion to get Polo to agree even to that."
"Oh," said Marco. "I didn't realize that dancing was a suitably refined profession for a lady of culture. Though I suppose it depends on the type of dance—" He looked at the pile of coconuts, then at the string bag. Taking it from her hand, he held it by the handles and tipped the coconuts into it, one by one. The bag made a tiny tinkling noise, which struck him as odd. When he stood up, the coconuts all fell out of the bottom.
"You idiot, Marco," said Elzabet. "That's not a bag."
[. . . .]
"That's not a bag, Marco," said Elzabet. "It's my costume." Marco said nothing but his eyes opened wide. Elzabet picked up the item. It was made of wide-meshed netting, and weighed perhaps half an ounce. She shook it, and the metal objects at the intersections jingled: bells. "The handles, Marco dear, are shoulder straps. And there's no bottom to it because my legs have to be able to poke out. It's a dress, my sweet: it is intended to contain the firm, youthful flesh of a desirable young woman.
"And not, you forsaken fool, seven hairy coconuts!"
There is an act featuring circus clowns whose limbs are outfitted with portal sleeves.
The audience roared with laughter. Snalikka, sitting about halfway back among the cheaper seats, roared with them. The khanatta had maintained an old circus tradition: the clowns. With syntelic variations, naturally.
One of the severed arms reached out, tripping up the second clown. The first clown reassembled himself, dragged the other over to the dog, and swapped their heads. The dog-head barked, while the first clown stepped back and nodded sagely, as if satisfied with the improvement. He was knocked over by a pink and yellow pantomime horse that galloped past with four naked human legs—one female.
gravity traps that look like water pools
"[. . .] Or that multi-part gravity shredder that got Sam's pony on the way to Two Mountains—"
"Ugh. That was horrible! The poor animal just wanted a drink—"
"—But the pool was the top end of a synte, the bottom end was hundreds of feet below, and the compressed gravity gradient pulled it through the moment its nose broke the surface. Then that same gravitational stress tore it to shreds, so the plant could feed easily. And that's another conservation law. Potential energy. If you lose height through a synte you gain velocity—or heat. Trying to go up against the gradient is like hitting a solid wall—except you can see right through it. There's an entirely new ecological device, obeying its own bizarre laws, and it's in everything. It makes Qish different from every other planet in the universe. And then to compound the effect—"
"About a thousand years ago the colony vessel Magog crash-landed on Qish and the would-be colonists woke up on a world where matter-transmitters grow on trees."