For Castiel, sex was something that he associated with feelings of love. Or, deeper feelings in some way. It was not something to take lightly and it was one of the reasons he wasn't ready for that. To experience that with Meg. Not because he didn't have feelings for her or wasn't sure of them, but rather he didn't know how she felt and he was also adjusting to the life he was now leading. It was not easy and adding the complication of sexual urges would only serve to confuse him more. But, hearing Meg tell him that she cared about him, it... brought on a swelling his chest, something that made him feel warm and bright, a feeling that he wanted to keep forever if he could. It was a good feeling and it caused him to smile, that smile he had for her, just a bit at hearing that. So, he made her happy. As happy as she made him, it seemed. That was news he rather liked to hear.
With that news, that flutter in his chest and the knowledge of how he had those urges for more with Meg, he didn't think of how he wasn't ready for this. For how he was also getting over an illness that, while it wouldn't spread to Meg, it was still a reason to go slow and steady rather than jump into things.
But he did, all the same. His hand moved from where it was at her hip, to tangle gently in her hair as he pulled her in closer to him. His lips met against hers, a softer, steadier manner and not frantic and passionate, but still firm and wanting all the same. He wanted Meg, wanted her to be aware of that, from the kiss he initiated to the slow realization of the press of their bodies together when he closed that barest hint of space between them.