In general, Luna isn't a huge fan of flying. It isn't that she finds it unnatural or anything that she's heard Muggles or Muggle-borns call it, or even that she is a afraid--the Thestral flight to the Ministry her fifth year was perfectly pleasant. She just prefers being on her own two feet. She tends to trust herself more than she trusts other things, such as other people's opinions, or wings that aren't on her back and part of her nervous system.
That said, when Severus finds it in himself to ask, "Would you care to ride Roghainn with me?" the last answer Luna is going to give him will be, "No."
She does say, "I thought that was something between you and her."
"For the most part," he agrees.
"I should just like you to join," is all he says.
Over the years she has learned how to interpret his silences. She knows some would see her interpretations as whimsical--or merely mad--but she finds them to be accurate enough between the two of them. This silence tells her nothing. "Very well."
"I would love to," she amends.
He looks at her suspiciously but seems to accept that she is telling the truth. They walk out to the stables together and she watches as Roghainn allows him to slip a bridle in her mouth for the sake of giving Luna some purchase. Roghainn is bigger than the Thestrals were, so she appreciates the concern. That said, she knows how to bounce.
He helps her mount and then draws himself up behind her, and oh, it was worth agreeing just for this moment of touch. It's not that he won't touch her if she asks, or even that behind closed doors there is a lack of touch. It is simply that every touch from him is new and distinct, and Luna enjoys each one as it comes.
He says, "Hold to the bridle."
She says, "You won't let me fall."
"Silly twit." The words are said softly into her ear.
She twists to kiss the side of his mouth before letting him concentrate, letting him say to Roghainn, "You're free to fly."