If this had occurred when he was still an angel, while he had no knowledge of the experience outside of when his lack of memories kept his angelic abilities dormant, he was certain this would have been different. It wasn't simply because he could have withstood the full force of her strength, but as witnessed from how she felt clean after he kissed her before, it was likely that his grace would have touched her in ways that his body could not. It would have reached out, touched her soul, as battered and torn as it was, and it would have been a lot more than just a sexual act between two willing parties. Now, however, he was just a man. He had a fragile, easily broken body and that was why his leading this through was the best, in learning each others bodies.
When her hips rolled into him, that groan from just before was nothing by comparison. While his arousal was nothing new, what came after it was. From the varied things that Meg was making him feel, making him aware of his body in ways he'd previously been in the dark about, he didn't care about muffling his reactions in any way, because it was nothing to be ashamed of, in his opinion. It was tentative, but thanks to the movement of her hips, he responded in a similar manner, just experimentally because he was uncertain of what was pleasing to Meg and what wasn't. Slowly, Castiel was sure he would find that out.
Between her body arching to his and that gasp, he studied her for a moment, taking this as a reaction that he most liked from her and knew it was because of his touch. It was but a moment later that he was bringing the bra away from her body, the satin moving against her skin as he drug it away. Slowly, carefully, his hands moved from the underside of her breast, where he started with just the lightest of touch, getting used to the feel of them against his hand. Aside from his time as Emmanuel and his being with Daphne then, Meg was the first woman he'd truly touched this way.