When he starts Hogwarts and is sorted into Slytherin, Lucius is the first person he meets. Lucius touches his shoulder in the Great Hall, and leads them to their dungeon common room, and helps him get to Potions class the next morning. Lucius has always been there, and when Severus grows older, he begins to realize what this meant.
It starts pretty early, once Severus is old enough to understand his desires. By then, Lucius is a 7th year; they only kiss once while they are both at Hogwarts. It is right outside the common room, but just a little farther down the hall, where darkness can conceal.
"This is fucked up," Lucius says, even as he bends down. Severus doesn't say anything. He is too busy concentrating, and remembering. Then they walk back into the common room together, and Lucius goes right over to where Narcissa lies on the couch, not one glance back at Severus.
That is in May, and after that many Mays go by before they even get a moment alone. Then one day, when Severus himself is a 7th year, he sees Lucius and Narcissa from a long distance in Diagon Alley. It is Christmas time, and he can see, even from so far off, that Narcissa is pregnant. He feels sick. They meet in the middle and shake hands all around, exchanging pleasantries. Severus mentions he's going to go have a drink before going home; being at home requires fortification.
"Oh Lucius, why don't you go with him? I need to buy your Christmas gifts anyway," Narcissa says.
"Yes, maybe I will." And he kisses her on her pretty cheek, staring straight at Severus over her shoulder.
They settle in at the pub and talk about nothing for a long time. Lucius talks about marriage, a little, and Severus tells him about the things that have and haven't changed at Hogwarts. A few drinks in, Severus feels brave.
"Lucius," he says, "there was something that happened a long time ago but that I still think about almost every day. And I wonder if you know what I mean, or if you care at all."
"Yes, Severus, I still think about that. But I shouldn't, and neither should you. Soon I'll have a baby, and you'll be out of school, with bigger things to worry about. Because there are bigger things. But you know that."
"Of course I know. But I do…I remember how it felt, how inevitable."
"What do you want me to do, Severus? Bend you over the table right here?" Lucius is hissing now, color rising.
"I think you want to," Severus says, bravery coming back.
"Dammit." Lucius gets up in a show of anger and leaves. Outside, Severus sees through the plate glass, he meets Narcissa. She smooths his hair and seems to ask him what's wrong, and he just shakes his head.
"Old grievances," Severus seems to see him say, and they leave.
Not really condoning Lucius and Severus kissing when Severus is ~13, just saying that their relationship is fucked from the start.