Thor has lain with women before and knows this routine well, enough to even tell that Loki is taking much more care in his beginning than most of his partners ever had... But it's surely because he's nervous.

Good thing he isn't nervous - his breath is picking up because of the magic in the air, the heaviness of the dim room around them, the feeling of could-be-anonymous fingers against his balls...

But Thor is too honest, and can't lie to himself anyone easier than he could to another. He knows it's Loki and can't help but think of dark hair slicked back...or perhaps falling out of place, locks curling rebelliously back up as Thor knows they are right now, as harried as he is. It's working, of course, the contact, and the thrill of the danger involved is perhaps only worsening (helping?) that fact - Thor feels himself slowly hardening even as he shifts with discomfort at the realization that despite the ritual being just business, he's actually thinking of Loki.

He shuts his eyes and thinks of Sif, although not without guilt.