Laura Cadman isn't easily shocked. She's a Marine - every filthy thing in the galaxy (okay, two galaxies) has been said either in front of her or directly to her face. Laura can hold her own with the big boys, and they respect her; for her sharp tongue, her sense of humor and for her comprehensive knowledge of explosives.
What she's seeing - through a conveniently placed interior window in the section of the city they're exploring - shocks her down to her toes. She knows her mouth has fallen open and that her hands are hanging by her sides. She feels as though all of the blood has drained out of her face.
It's...them. In...there. They're in an empty room. Laura would bet real money that the door is locked. She'd double down that they didn't notice the window. All that blood that drained from her face comes back as she watches, she can feel her cheeks flaming.
A good Marine probably shouldn't watch her C.O. let McKay strip his BDU pants down his legs, but Laura's never really been all that good. Or all that good at denying herself. And Laura wants to see this. When she was in Rodney's head, she'd watched his fantasies unspool - it was like watching a movie without the popcorn - and Sheppard starred in most of Rodney's Best Picture nominees.
Laura's pretty sure Sheppard's not acting. Sheppard looks like sex when he's just standing around, so Sheppard having sex looks like porn. High-quality porn. And, surprisingly, so does Rodney. When Sheppard throws his head back, Rodney pauses and gives him a look that Laura can't quite pin down. Lust, yes - a touch of amazement - but there's something else there, something softer, and it's nice to see. Despite herself, Laura likes Rodney, even when he's a complete prick.
She sees Sheppard's eyes flutter shut when Rodney's mouth touches his neck. She knows this isn't the first time they've done this, because Rodney knows exactly where to bite so that the mark won't show above Sheppard's collar. They're gorgeous together, even when they only manage to strip halfway, and they're clumsy and they get caught up in their own shirts.
Maybe it's the smile on Rodney's face, and the answering one on Sheppard's, but - whatever it is - it makes her happy. She watches a little bit longer, but when Rodney turns Sheppard around and pushes him over the table they'd been leaning on, she moves away and walks a few steps down the corridor to stand guard.
She's not actually watching out for them - she's just recovering from the shock.