I think this is probably Dinosaurs by Walter Jon Williams. I read it in the anthology The Furthest Horizon edited by Gardner Dozois, and I believe it was also published in Asimov's Science Fiction June, 1987.
The diplomat, Drill, is described as being eighteen feet high rather than seven metres:
Drill stood eighteen feet tall on his two pillarlike legs, each with a splayed foot that displayed a horny underside and vestigial nails. His skin was ebony and was draped in folds over his vast naked body. His pendulous maleness swung loosely as he walked. As he stepped across the open space he was conscious of the fact that he was the ultimate product of nine million years of human evolution, all leading to the expansion, diversification, and perfection that was now humanity's manifest existence.
The sexual relief isn't exactly provided by the ship, but by a semi-sentient being on the ship:
Surrogate lived in a dim, quiet room filled with the murmuring sound of its own heartbeat. It was a human subspecies, about the intelligence of Lowbrain, designed to comfort voyagers on long journeys through space, when carnal access to their own subspecies might necessarily be limited. Surrogate had a variety of sexual equipment designed for the accommodation of the various human subspecies and their sexes. It also had large mammaries that gave nutritious milk, and a rudimentary head capable of voicing simple thoughts.
The aliens are called Shars. The aliens didn't attack a human planet, it was the other way around. Human terraforming ships attacked a Shar planet without realising it was inhabited:
The Shars were still for a long time. "Your species attacked our planets, Ambassador. Without warning, without making yourselves known to us." Gram's tone was unusually flat. Perhaps, Drill thought, she was attempting to conceal great emotion.
"Yes," Drill said. "But those were not our military formations. Your species were contacted only by our terraforming Ships. They did not attack your people, as such—they were only peripherally aware of your existence. Their function was merely to seed the planets with lifeforms favorable to human existence. Unfortunately for your people, part of the function of these lifeforms is to destroy the native life of the planet."
However it does end with the military on their way to destroy the Shar. The Shar refuse to believe that the destruction caused by the terraforming ships was accidental and they respond by attacking human worlds. The final conversation between Drill and the Shar leader is:
"If Vang attacks any human planets after the Memories of the peace arrangement have arrived," Drill said, "the military will be unleashed to wipe you out. There is no stopping them after that point."
"How long," she asked, "do you think we have?"
"A few years. Ten at the most."
"Our species will be dead."
"Yes. Our military are very good at their jobs.