Snape took a deep breath, saying, "A long time ago, well, right here, actually... There were two wizards and two witches. The four Founders of Hogwarts. They had come together to create a school of learning, or so the story goes. Reality casts a different light on the picture - in the Middle Ages, there were spates of witch-burnings."
Snape looked at Hermione (who was not twitching with the need to interrupt), and said, "Yes, I do realize many of them were actually Muggles."
He took a deep breath and continued, "In those times, there were many orphans - and then, as now, the Wizarding world has always believed in Family. Not in the sense of love and affection, but in the sense of belonging. You are entrusted with your family honor. Your family holds your allegiance above all else."
"Orphans were considered disruptive - a truly unfettered person has little to hold him within bounds of proper decorum or societal obligation." Snape gave a sniff, as if he was saying that Gryffindors, in general, didn't know societal obligation from bears.
"To take a different example, an unmarried witch was considered disruptive because she was likely to incite fights amongst wizards, whether she willed it or no." Snape said sternly.
That. That was too much for Hermione, who gaped, and then boiled over, "But that's the wizards' fault, not hers at all!"
"It would be difficult to write a rule to stop men from flirting over unattached women, if you hold ostracism of the recently widowed to be offensive." Snape purred in his honeyed tones.
Hermione seemed to pause, her bright eyes focusing on Snape's face, as if telling him that she was scrutinizing his argument for any weaknesses at all.
"How much easier, in a legal sense, to ensure that all adults are properly bound in the confines of marriage?" Snape said. "Though unattached women were considered more problematic, men were not allowed to be unattached either."
Hermione, begrudgingly, nodded.
Harry wanted some popcorn. It was typical of Hermione to want to put centuries' old morality on trial. Not quite so typical for Snape to defend it, so Harry slowly made the decision that Snape might not mean everything he said, here and now.
"What is the purpose of marriage?" Snape prompted.
Harry blinked, he'd been willing to just sit back and listen to the story.
"For two people who love each other to celebrate their livelong intentions, and to confirm their vows within a legalistic framework." Hermione said. Harry nearly smirked a rueful smirk, only catching it a split-second before it was out the door. Hermione was picking up on Snape's wording.
"In medieval times, many marriages were arranged. In the Wizarding world, much more than the Muggle one." Snape said, "As the Wizarding families were considerably more affluent." Snape looked at them both, "What was the purpose of marriage?"
"Caretaking of children, sir." Harry put out, "With the Black Death, it was unsafe to assume that one person would be able to survive long enough to see her children to adulthoood."
Hermione had the textbook answer, "To bind together families with blood ties."
Snape nodded, slowly, "Both are correct, and neither, as is often the case."
He turned, walked away from them, and then pivoted, returning without saying a word. "The founders graciously decided to extend their Houses to cover those of similar mind, as decided by the Sorting Hat. It was to be a House of Last Resort; particularly, for orphans, a place to belong, with likeminded individuals whose strengths would complement each other."
Snape spun on one heel, not saying a word until he'd returned to looking at them, "Over the long years since, the custom has changed somewhat. Those who are rejected from their Houses, can, at least, claim membership in their Founder's house. The Housemasters at Hogwarts have the titular role for the Founders' Houses, standing in for the Lord. As such, they have certain decision-making powers that would otherwise devolve to the child in question."
"Like what?" Hermione prompted.
"Supervision of contracts, arbitrage with other Houses, representation of Parents during Marriage Ceremonies, Next of Kin where appropriate..." Snape let his words trail off. "It was why the Dark Lord's announcement that he was the Heir of Slytherin drew so much interest from those of my house."
"But... aren't his followers mostly pureblood?" Harry asked, puzzled.
"Yes." Snape said, "They were not lured by promises of wealth and status alone. If their allegiance to the Dark Lord was discovered by their families, he promised that they'd be formally adopted into his own house. Whether he could have made that happen is an open question, of course." Snape said, "But it made it easier for Slytherins to defy their Lords, to know that they would still have a Family and a House." It was impossible to hear the words without the capitalization.
Hermione twisted her hands behind her back, before asking, "Sir, you said that you were a member of Slytherin House. What does that mean?"
Snape's mouth twisted, but it wasn't what you'd term a smile. More a wavy line, with a compressed middle. "It is considered a major conflict of interest for a Headmaster to have a personal House. You will find that the other Heads of House are half-bloods. It may surprise you, but the Headmaster of Hogwarts is traditionally a position where having a personal House has been considered an asset."
[a/n: grumble. Please review, I'm having a bad day.]