Severus knew enough Celtic witchlore to know that red hair and green eyes are a visual code for fertility goddess. Some potioneers invoke them. Nantosuelta, Damara, Onuava, Rosmerta: they came into play for contraception, forcing blossom, fertilizing, and brewery. It was all very mixed up. They'd forgive a slip of the tongue, bless you at least a bit even if you asked them the wrong favor. But it was a witch's thing. Magic for the disenfranchised. Someone like McGonagall, more than equal to any man, wouldn't think about it. She'd ignore this back alley of magic like she'd ignore Knockturn Alley, like her namesake ignored the Furies (women's goddesses through and through) in the Oresteia, a trilogy which most muggles didn't realize had magic properties.
Even when it focussed on decidedly male gods, the old religious magic remained a witch's thing. Severus had seen his mother murmur for the Green Man when she watered her potted plants; he'd never seen a wizard-Slughorn, say-call even on Cernunnos. And sometimes, Slughorn needed the help. It did make Severus feel a little better that, if he had to be left out of the Slug Club, Cernunnos was left out too.
Once, Severus's mother told him what sounded like a witch's secret: that Oestre was the most powerful invocation of the era. Still useful for pregnancy magic (which often involved rabbits) or making almost anything young for a season, Oestre had picked up some associations with Easter, the holiday that hijacked her name. Now that name was all tangled up with self-sacrifice, and love.
Self-sacrifice. For a while in NEWT potions, he'd played with the idea that self-sacrifice was a powerful catalyst for magic. That maybe, for instance, truly giving yourself into a euphoric elixir would make it induce actual happiness. Or turn Amortentia from an infatuation draught to the first true love potion. But he'd been too busy with the Death Eaters to fully pursue that line of thought.
But damn if Lily didn't grow every day into a new Oestre. Hair like the scarlet pimpernel.* Eyes like grass or glass or April. A body that Severus described in his guiltiest thoughts as lissom, or slender, or blooming. Lily would snort if he told her all this; her mind was too sharp, too quick to be Oestre-like. But her soul was appropriately soft.
Severus whispered to Oestre when he made his third Elixir to Induce Euphoria; he did his best to feed the potion a rare memory of touching Lily's hair. He also added a sprig of peppermint. He drank the whole bottle on the first day of summer and was high for days. Hard to be sure whether the herbs or the prayers did it. But, as his mother once told him, nursing her own black eye, goddesses work in mysterious ways.
*Anagallis arvensis, whose flame-colored blossom is often known as Shepherd's Clock. I imagine it is necessary in time-turners, which Baroness Orczy's hero made extensive use of. In wizarding circles it is well known that the book was written to cover up the activities of a British halfblood rescuing aristocratic wizards and witches from the guillotine, which isn't as easy to survive as a spot of burning at the stake.