[Reid's well aware that Rebecca's previously had problems with boundaries, just plainly unable to see them here and there. (Sex toy baskets being A Thing.) But once she's aware of them she does try to make an attempt to respect them, which is why she's treating this situation a lot more tenderly than she would any other guy. Desmond or Shaun would've just gotten a playful shove, or just as simply she would've helped them out over it. She knew them, and with them that was cool, but with Reid there's more thought involved.
And maybe it took this situation, the removal of Kitty from the situation for Rebecca to remember Reid's not a bad looking guy. (And she does miss Kitty, doesn't want to be any sort of in between that, but... Kitty's not here. It's just the two of them, literally, and quite possibly forever in the future. It's not Marina anymore.) And she is kind of lonely in a lot of ways, clinging to Reid in particular for being there.
Still somewhat over him, she hasn't sunk back down to lay against his side nor has she made any further motion to get up. Limbo between the two, she keeps eye contact in silence. It's a moment where gravitation happens or it doesn't, evident by her body moving just enough to line up with his but not enough to dip entirely down.]