Castiel captured that sigh as his own when he kissed her, taking that softness and continuing on with it. When he kissed her before, after the pizza man, it lacked this softness, that need to actually connect to her. He wasn't simply kissing her, he was trying to connect and feel something, to understand what was going on here between them. And the fact she responded in kind, it made his heart beat faster against his chest, because he was now less apprehensive about how far Meg's affection for him went.
When she did pull away, there was a quiet groan in his throat at that, because it was sooner than he wanted, too. It wasn't indulgence, it wasn't exploration, but rather need. Want. It might be keeping them in that safety zone, keeping him from pressing to that point of no return he couldn't turn back from, but it was a zone that was a nuisance in this moment than anything.
"I... hadn't planned on returning to my apartment tonight." He admitted, looking down for a moment, not quite out of shyness, but more because he hadn't planned ahead to begin with. He just wanted to see Meg and didn't think too far beyond that. "I can sleep on the couch." Not that she used the bed, because she didn't sleep, but he didn't see any issue with the couch. It was comfortable enough.