Kabuto doesn't think that Orochimaru knows how to love.
He's good at inspiring love. Very good. There's a literal army of people who are taken with him, and a string of broken hearts lining his life's path; Jiraiya, Anko, Sasori, to name major ones. And Kimimaro. God, it had been nearly painful to see how desperately Kimimaro had fallen. Now even Sasuke's starting to get starry-eyed in the sannin's presence. Of course Kabuto himself is nowhere near immune to his master's magnetism; sometimes he thinks he's in deeper than Kimimaro, in ways.
Because Orochimaru is also good at imitating love in the physical sense. Better than anyone else Kabuto's ever known. God knows how many hours he's spent screaming himself hoarse beneath the skilled touch of his master. It is Kabuto's personal belief that the gods created Orochimaru with the ability of giving mankind the greatest pleasure it could comprehend. And he's in love with it; in love with the gentle caresses and poison-dipped whispers, with the way his master can make him feel more important than anyone else in the world.
But Kabuto also believes that Orochimaru's long since lost his capability to care for anyone but himself. That where normal people have a heart, Orochimaru has a shriveled-up pile of ashes. That no matter how many times their bodies twine together beneath the sheets, and no matter how horribly much Kabuto wants it, Orochimaru will never love him.
Which is why he's shocked when those taboo words slip into his ear while he's feigning sleep after sex.
"I love you," whispers the sannin, pressing a gentle kiss to Kabuto's shoulder, and it's obvious that he isn't trying to be heard; that he thinks Kabuto is unconscious and the words haven't really been shared.
But Kabuto hears.