This was inspired by the first few paragraphs of Chapter 27 of Order of the Phoenix. Rowling pointed out the path, I merely followed to see where it would lead. Really, I don't see how it could have gone anywhere else.
"I think having Firenze here has been a force for morality," said Trelawney. "I've been following his lectures - as we're both teaching Divination, I don't want to repeat material - and I've watched the students. They sit in a circle on the 'forest floor' of that glade Dumbledore's made out of Classroom Eleven, and he prances around in the middle as he lectures. They can't help but stare, boys and girls alike. Firenze would be hard to overlook, even if they weren't all seated at eye level."
"I fail to see how that would promote morality," said Madame Hooch. "Quite the opposite, I'd assume."
All eyes turned to Trelawney. She opened her mouth to speak, and suddenly giggled. "He may have four legs, but he's got about two feet. The girls, poor things, imagine what they'd be getting into, or rather vice versa. And the boys seem awed, and intimidated."
McGonagall nodded. "They're insecure and sensitive at that age. They're not willing to have the girls compare them to Firenze. So they avoid intimacy."
Wilhelmina Grubbly-Plank coughed delicately. "Several girls have asked me about interspecies breeding. They start with half-giants, or half-Veelas, and they always work their way around to centaurs and humans. And this morning I visited Blotts in the village and they asked if I were teaching unicorns or horses this term. They'd sold out of curry combs this last Hogsmeade visit, and wanted to know if they should stock up."
"Is such a thing, well, even possible?" asked Trelawney, "It seems, ah, a bit of a stretch."
"Come now, you've all been to Ladies' Night at the Three Broomsticks when the landlady's son dances. We've been as a group whenever one of the younger teachers gets married. He's living proof it's possible. His father was from the same herd as Firenze."
"I'd assumed that the mane, the ears and the tail were an illusion he used to go along with, well, the other. Like a costume."
"Quite genuine," said Pomfrey, "I was called in to treat him frequently when he was little, as was Hagrid. He had all the usual diseases of children, and of foals too. I remember the poor child had persistent digestive disorders as a teenager, until I learned his mother had been confused by some Muggle campaign to 'keep kids off grass'. She finally understood his system needed the roughage."
Madame Pomfrey spoke up. "Several girls have asked me about the question of, ah, size. The practicality of it, and the what cure is available or what preparations to make."
McGonagall looked around the table. "I think we'd all like to hear the answers to those questions, Poppy."
"It's not as serious a problem as you might think. However large Firenze may be in comparison to a man, he's small in comparison to a baby. So even without any magical assistance, with proper preparation the thing is possible. Not enjoyable, perhaps, but possible."
"What are the risks? The remedies? What preparatory measures can you recommend?"
Pomfrey looked around the table, slowly. "I'm obliged to keep medical confidences, and you've all been glad of that at one time or another." The assembled witches shifted in their seats and avoided meeting each others' eyes. "That said, you all know that Umbridge was in the infirmary for a month after her evening with the centaurs.
"There, an entire herd was involved, as well as the use of force. Now, rape isn't sex, it's violence. I needed to use Phoenix tears - 'tears for tears' as we say. Even so, she was up and out by term's end.
"In a case of romance, rather than rape, it is a far easier matter. A few applications of Mungo's Marital Miracle cream, some Eastern exercises . . . it's not really a new problem. I see it after childbirth, and where someone's dated Hagrid.
Several staff members looked up. Several others looked down.
"As to preparation, there's a variation on the Alhoamara door-opening spell, known as the ' Oh Holy Mary! ' spell."
"Wilhelmina, did you get a list of the students who got currycombs at Blott's?"
"Yes, I thought that might be helpful. Here it is. Six girls, and one boy."
"A boy?" McGonagall turned to Pomfrey and raised an eyebrow.
"It's hard to evaluate the risks, without knowing just what he had in mind. If I could have the list?" McGonagall passed the paper. "Oh yes, I've seen him in hospital before. I'm sure he'd put this behind him easily."
"Well then, have a quiet talk with each of them. One by one, I think, a group session could be awkward. They're all interested in one centaur, after all."
Madame Hooch spoke. "As you just said, Minerva, there's only one centaur here. Wouldn't it be easier to talk to him, and make him behave, than to try to control all these girls, er, students?"
"Well, that's the crux of the problem. When Firenze came to Hogwarts to teach he was expelled from his herd. Hogwarts can scarcely expel him for misbehavior. He has simply nowhere to go.
"Plus, you must understand that centaurs aren't humans. They view these things much differently than we do. They have strong ethics, but they don't have human morality.
"Firenze would consider an order to remain chaste as reasonable, and as moral, as an order to remain constipated. For a centaur chastity and constipation are equally lacking in moral importance. They're merely uncomfortable failures of normal physiological functions."
"Well, he's expelled from his herd. Are horses an alternative, or thestrels?"
"Can I set you up Saturday night with a nice orangutan, Sybill?"
"Minerva! That was uncalled for!"
"Perhaps. But now you understand why Firenze might not react well to the idea of a candlelit dinner with a mare."
"So hard to find a hotel with a bridle suite," murmured Hooch.
Trelawney said, "Well, I can't See an answer. Centaurs are out, horses are out, and I'm sure we agree students are out, even if they are, ah, hot to trot. Who does that leave?"
McGonagall coughed. "Dumbledore asked me to call you all together, and put the question to you. He said we were equipped to resolve the situation. He asked me to remind you, and these are his words, 'It is a Hogwarts tradition to accommodate a member of the faculty.'"
"How are we equipped? Why us, and why isn't Dumbledore here himself, or Hagrid, who best understands centaurs?"
"Albus asked me to call together the female staff of Hogwarts. If you consider in what way we are equipped that he and Hagrid are not, I think that will tell you what solution Dumbledore anticipates."
"I'm not sure I understand you, Minerva," said Trelawney, "That Dumbledore would suggest . . . it seems a bit much to swallow."
"Spit, then," said Hooch, under her breath.
McGonagall held up both hands, palms outwards. "I had thought of asking for volunteers. Nearly all of you are single. Some never married. The dedication that magic requires leaves little time for romance. Others are widowed. The war with Voldemort was hard on the ranks of wizards.
"But this is a sacrifice for Hogwarts. So I'm going to offer anyone who feels unable to make that sacrifice the chance to be excused, and the rest of us will draw straws. The long straw resolves Firenze's . . . situation.
"Hooch, you wish to be excused? After seeing you buying lap dances at the Hog's Head, I must say I'm surprised."
"No, no, Minerva, of course I'm as willing as the rest to make the sacrifice. I'm just wondering if it's not unfair to place the entire burden on one of us. Wouldn't it be fairer to take turns? Month by month, perhaps?"
"Or even week by week," piped up Sprout.
"I have no Thursday classes, and could take every Wednesday night," said Hooch.
"Wilhemina, what do you think?"
"Well, from the waist down, he's a horse. Horses mate when a mare's in season. One stallion can top a whole herd. So in coming up with a scheme we need not worry about overtiring Firenze."
"Why, even that half-centaur boy at the Three Broomsticks has an amazing resilience . . ."
". . . or so I've heard," concluded Hooch, suddenly self-conscious.
McGonagall looked around the table. "Does alternating nights seem the fairest scheme to everyone then? Good. I'm glad to see so much school spirit. Poppy, I'm going to ask you to set up a schedule. With your medical experience - and your undergraduate experience - you seem the best choice."
Trewlaney stood. "Ladies, I See a much happier year, a more filling, I mean fulfilling, year ahead at Hogwarts. I never thought I'd say this. But I'd like to propose a toast to the one responsible for adding Firenze to the faculty. Here's to Dolores Umbridge."