I think early 1980s, though it might be a bit earlier. I'm almost certain I read this in Omni.
"The Far Side of the Bell-Shaped Curve", a novelette by Robert Silverberg, was first published in Omni, March 1982, starting on p. 56 (it's listed on the contents page by a different title, "The Dangers of Time Travel"); a scanned copy is available at William Flew. Here is a summary from Majipoor.com:
Reichenbach is a time traveler from some year after 2187, jaunting to the assassination of Archduke Ferdinand, where he meets Ilsabet, a fellow time traveler. They fall in love and jaunt through the ages, visiting the great moments of history (mostly disasters), until old-fashioned jealousy rears its ugly head.
The premise of this one is that technology has made it possible for people to travel back to different eras in Earth's history, through the use of a small device mounted on the belt.
It's called a timer:
She wore her timer high, a pale taut band just beneath her breasts.
This device may also provide some measure of ability to fit in/speak the language/etc. of the era the traveler is visiting.
Not a function of the timer, that's provided by "halfway houses" in the past:
At the halfway house outside imperial Rome, they underwent their preparations, receiving their Roman hairstyles and clothing, their hypnocourses in Latin, their purses of denarii and sestertia, their plague inoculations, their new temporary names. He was Quintus Junius Veranius, she was Flavia Julia Lepida.
The only hard and fast rule is, a traveler may not visit the same time period twice, as there could be paradoxes involved.
"We aren't supposed to reenter a time-span where we're already present. There must be some good reason for that. The rules—"
[. . . .]
"Because I don't know what will happen to me if I do. Kinky and quirky it may be to pile into bed with our other selves, but something about the idea troubles me, and I dislike needless risk. Do you believe you understand paradox theory fully?"
The story is told through the viewpoint of one such traveler, a man who has fairly recently begun traveling with an attractive woman he met in one of his time jaunts.
Under her deft consolations the sting of his oafishness in the Mermaid Tavern eased, and his mood brightened as they went onward. Few words passed between them: a look, a smile, the merest of contacts, and they communicated. Attending the trial of Socrates, they touched fingertips lightly, secretly, and it was the deepest of communications. Afterward they made love under the clear, bright winter sky of Athens on a gray-green hillside rich with lavender and myrtle, and emerged from shivering ecstasies to find themselves with an audience of mournful scruffy goats—a perfect leap of context and metaphor, and for days thereafter they made one another laugh with only the most delicate pantomimed reminder of the scene. Onward they went to see grim, limping, austere old Magellan sail off around the world with his five little ships from the mouth of the Guadalquivir, and at a whim they leaped to India, staining their skins and playing at Hindus as they viewed the expedition of Vasco da Gama come sailing into harbor at Calicut, and then it seemed proper to go on to Spain in dry, hot summer to drink sour white wine and watch ruddy freckled-faced Columbus get his pitiful little fleet out to sea.
There is a passage about the two sharing stories of their sexual exploits. The woman particularly enjoyed a fling with a burly Scotsman who liked to sing loudly during intimate moments.
No Scotsman, you must be thinking of the Swede:
Of course, they took other lovers from time to time. That was part of the game, too tasty a treat to forswear. In Byzantium, on the eve of the Frankish conquest, he passed a night with a dark-eyed voluptuous Greek who oiled her breasts with musky mysterious unguents, and Ilsabet with a towering garlicky Swede of the imperial guard, and when they found each other the next day, they described to one another in the most flamboyant of detail the strangenesses of their night's sport—the tireless Norseman's toneless bellowing of sagas in his hottest moments; the Byzantine's startling, convulsive, climactic fit, almost epileptic in style, and, as she had admitted playfully at dawn, mostly a counterfeit.
The relationship (such as it is), begins to sour when they encounter another time traveler.
Reichenbach did not fail to notice the rebirth of light in Ilsabet's eyes, and the barely perceptible inclining-forward of her body as the stranger approached their table was amply perceptible to him. The newcomer knew them for what they were, naturally, and invited himself to join them. His name was Stavanger; he had been on his jaunt just a few days; he meant to see everything, everything, before his time was up. Not for many years had Reichenbach felt such jealousy.
IIRC, he tries to have the rival poisoned while they are all in Italy during the Renaissance. The poisoner took the money and left the city, causing the plot to fail.
In Borgia, Italy, Reichenbach hired a Florentine prisoner to do Stavanger in with a dram of nightshade. But the villain pocketed Reichenbach's down payment and disappeared without a care for the ducats due him on completion of the job.
The ending is not in the Spanish Inquisition but the French Reign of Terror:
Too late. They had his arms; the other Reichenbach was groping for his timer, seizing, tearing. Reichenbach fought ferociously, but they bore him to the ground and knelt on his chest.
Through a haze of fear and pain he heard the other saying, "This man is the proscribed aristocrat Charles Evrémonde, called Darnay, enemy of the Republic, member of a family of tyrants. I denounce him for having used his privileges in the oppression of the people."
"He will face the tribunal tonight," said the one kneeling on Reichenbach.
