Mark
The second you chose to forego the other end of the couch and instead sit on Mark's lap, he'd wrap his arms around your midsection as if it were second nature. To him, it just about was; this was a common occurrence between the two of you, regardless of if you were alone or had company (within reason, of course). He'd reflexively rest his head in the crook of your neck, breathing in the familiar scent of you. He enjoyed simple moments like this one with you, moments that brought peace to him.
Jaebum
With Jaebum, this was more often than not a calming tactic. When you could see he'd worked himself to a temper, or simply seemed overly stressed, you'd give him a few moments to himself, following after him once you felt it was the time to. Wordlessly, you'd lower yourself down on his lap, wrapping your arms around him, hoping he felt as safe and loved in your embrace as you did in his.
Jinyoung
Jinyoung was never opposed to physical affection as far as you could tell, although he seldom was the type to initiate it. This being said, you being on his lap was an exception; he simply loved it. Everytime the two of you would find yourself alone, he'd pull you to him, setting you onto his lap. Not that you minded at all; though you'd never admit it to him, you adored that he did this.
Jackson
With Jackson, these moments happened rather randomly. They weren't particularly common, and yet when they did happen, you were never surprised. However, this never really stopped there being butterflies in your stomach that took flight every single time he pulled you to him. And him? He loved the feel of having the most precious person to him pulled up close, feeling the same kind of joy he was.
Youngjae
Youngjae just appreciated the torrent of skinship. While he's the "purest" of the lot, sitting on his lap was never really a sexual thing; it was always a I-find-comfort-in-you-lets-just-be-like-this-forever type thing. Lazy movie night for the first time in a while? You're on his lap. Music composing or lyrics time? You're on his lap. It was just another one of those simple but beautiful little nuances of being with someone who you would love for the rest of your days.
Bambam
Bambam often would have you like this prior to leaving you for a little while to do his concerts overseas. He was normally very physically affectionate, and happily so, but times where he really wanted to cement in your mind just how much he loved you, he preferred having you sat on his lap, where he could hold you, and hopefully reassure you. It was an intimate thing to him, and that he chose to do this with you of his own accord meant something. It meant a lot.
Yugyeom
The world is full of men of different shapes, sizes, traits, and Yugyeom would hardly fail to notice. It wasn't something he minded, until the irrational fear of one of those combinations being more appealing to you than him would pop into his mind. Him being the youngest of his group, it wasn't too unusual for him to feel insecure about these sorts of things, the only solution of it being you plopping down on his lap and lovingly telling him how wrong he was. You loved him, and that was that. nothing less, nothing more.