The key, Regulus decides, is to let Lucius think every moment of this night is initiated by him. Regulus must remain blameless. It cannot appear as though he had any notion of wanting this before it has happened.
So he is patient. Dinner conversation is no more laden with suggestion than usual; the after-dinner chess match results in Regulus' win, something that has been happening surprisingly often of late.
It is not until Lucius walks Regulus to the guest quarters at nearly two in the morning, as has become his custom, that Lucius finally makes a move.
Regulus, as usual, lingers outside the door, finishing one conversation only to lead into another.
"You still haven't mentioned anything about new nieces and nephews. And I'm still intensely curious about why 'Cissa was so defensive about it."
Lucius looks at him for a long moment, then backs him quickly against the door to the bedroom.
Regulus blink, trying for the most sincerely innocent look he can manage. "Lucius…?"
"This is why," Lucius says, voice low and almost growling, and he presses his lips to Regulus'.
Regulus allows himself a moment of frozen surprise, to make things more realistic, and then kisses back, threading his fingers in Lucius' hair.
Lucius Malfoy, truly, is a phenomenal kisser. Regulus takes note of the way soft lips slide with his, the deft tongue that tangles with his own, the way Lucius' body pressing him against the door makes him feel more safe than trapped. Luciu's hair is like silk between his fingers, Lucius' hands gentle as they cup his face.
When they break apart, Regulus' eyes open and find a truly novel sight. He has never imagined, even when he came up with this plan, that he could reduce Lucius Malfoy to the equivalent of an inexperienced schoolboy, and yet he is red-faced and panting and looks as though he is fighting a grin.
"Mr. Malfoy," Regulus murmurs, "would you like to come inside?"
"I would love to," Lucius breathes.
"All right then, question number two: would you like to come into the bedroom?"
Lucius stares at him for a moment, then laughs. "You are too young to be so well-versed in innuendo."
"There you go bringing up my age again," Regulus whispers, leaning up on his toes to let his breath ghost over Lucius' ear.
"I've told you before," Lucius replies, "that youth is, as you put it, a crippling disease."
"Well," Regulus says, pushing open the door and pulling Lucius inside by the hand, "I can promise that this is another thing in which I am far from incompetent."
"We shall see," Lucius says and nudges the door closed behind them with his foot.
The rest of the night will be remembered by Regulus as a blur. A pleasurable one, certainly, as Lucius is as excellent at sex as he is at kissing, but a blur nonetheless. He remembers only vaguely the burn of being entered, the push and the pull and the shockwaves of pleasure every time Lucius hits that place inside him.
He doesn't remember falling asleep tucked against Lucius' chest, but he wakes up early the next morning with his face nestled against bare skin. He presses his lips to the area he can reach, tilting his head up to see a nipple and swiping his tongue over it. Lucius shudders, and a hand in Regulus' hair tilts his head back. Lucius' lips find his quickly, then trail down his chin and to his neck.
"This is a nice way to wake up," Regulus says, biting back a moan as Lucius nips at the skin of his collarbone. "Hey. Hold on for a moment, I want to talk to you."
Lucius pauses, sits up. "Hm?"
"You are married to my cousin."
"...Yes, I'm aware of that, but thank you for reminding me."
"What I mean," Regulus says impatiently, "is that — I just want to know what this is to you. A one-night stand or..."
"...No." Lucius drags his fingers through his white blonde hair. "Unless you want it to be."
Regulus is sent into a fit of giggles he can't seem to hold back. "You look thoroughly undignified right now, it's rather adorable."
"Oh, shut up."
Once he's sobered up, Regulus pushes himself into a sitting position, hands lingering uncertainly in his lap. "It's not a one-night stand to me either. I don't want 'Cissa to get hurt, though. And I — " Regulus purses his lips. "I don't know how much respect I can have for myself, fooling around with a married man."
"Dear Regulus," Lucius says, stroking the pad of his thumb over Regulus' cheek, "someday, when your mother starts to harass you about marriage, as mine did to me, you will find a pretty trophy wife amongst the ranks of the Death Eaters, probably, or one of their sisters, and you will marry because it is what is expected of you as the heir to the Black family. Do you find yourself attracted in any capacity to women?"
"Not particularly," Regulus says truthfully.
"I trust you will not live a life of celibacy simply for some misplaced morality about marriage. Narcissa is fully aware that our marriage is one of obligation."
Regulus contemplates this for a moment, fingers playing with the hem of the sheets that drape over his lap. He remembers when he was fourteen, listening at his bedroom door as his mother screamed and Sirius screamed right back, because she'd found letters, hidden in a box under his bed, letters that "bordered on pornography," his mother had said, and how could Sirius shame the family by fucking a half-blood, a werewolf, but worst — worst, Remus Lupin was a boy, and the Black family heir could not cavort about in bed with other men.
That was the night Sirius left for good.
Regulus glances up at Lucius. "Is she really?"
"You asked why she got so defensive at dinner. It's not as if we sleep together often. I can't..." Lucius shakes his head. "It's not only me, you know. She's been with other men since we've been married. It's what we have to do. If I can find a way to have my cake and eat it, I'm going to do it."
"You are being shockingly honest this morning. I think I like you post-coital and drowsy." Regulus' fingers stroke over the back of Lucius' hand. "My mother cannot find out about this. Not under any circumstances."
"Do you know, I was just thinking of flooing her to let her know." Lucius rolled his eyes. "Honestly."
"It's why she threw Sirius out of the house. He got burned off the family tree for being with a man."
"That's hardly the only reason. I presume it was the cherry on top of all the other things he's done to make your mother angry. Besides, it was Lupin, wasn't it? The half-breed."
Regulus fights a wince at the term, at the way it rolls off Lucius' tongue so easily. Remus Lupin, Regulus knows, was one of the few people in the world who could keep any sort of leash on Sirius and his friend Potter, and he was always in the top few students of the class, and he was quiet and bookish and everyone liked him who got a chance to talk to him.
Lucius would never care about anything like that. To Lucius, any werewolf, no matter how kind, will never be more than a "half-breed."
"Lupin, yes," Regulus says instead. "Sleeping with a Malfoy is certainly far more acceptable than sleeping with a werewolf. I suppose it's true that as long as I promise to marry, she wouldn't be that furious with me."
"Well, it's not enough to marry, you've got to promise them boatloads of grandchildren to keep them satisfied." Lucius smirks. "But as long as you manage one brat, you could probably get away with not shagging your wife for the rest of your life."
"Honestly," Regulus says. "Sex has the strangest effect on you. You realize you're using slang."
"Was I? I'm terribly sorry, shall I clean up my language?"
"I'm not sure. It's terribly disconcerting, but I think I sort of like it." Regulus grins. "I wonder if a few more rounds could reduce you to incoherency."
"I sincerely doubt it," Lucius replies.
"Yes, but what an interesting experiment it would be, don't you think?" Regulus doesn't wait for an answer before he tugs Lucius toward him by the arm, bringing their mouths together once more.