Miranda is Shepard.

What you see and hear is Miranda in front of a full body mirror, masturbating to her own image. Only according to her words it's not her image.

''Ohhh, fuck! I wonder if Miranda loves her new body as much as I do mine.'' You hear 'Miranda' moan followed soon by a reveal of who is now in Miranda's body.

''Maybe someone's caught on. She's a genius, but not the only genius on the Normandy. Hmm, Liara and the new me, that's a hot thought, I'll have to think about that more.''

So Commander Shepard and Miranda Lawson swapped bodies. You wonder how and more importantly when this happened. You also wonder what it means for your mission. If you **** her with this information you would actually be blackmailing the Commander instead of Miranda after all. Maybe you should ask Aria about this before taking action, but that could void the opportunity.