It was an unusual feeling for Margaery not to be looked at with desire, she was an attractive woman and she knew it. She also used this knowledge to get her own way, since men were so terribly easy to manipulate. With Renly however it was very different.
Renly had never looked at her in that way, the reason being clear to her now of course. Not that it bothered her, she wasn't a young girl in unrequited love with her husband. She was a woman who wished to be queen and knew what she had to do in order to make it happen. Her only concern was that Renly would be unable to do what was needed but they would cross that bridge when they came to it. He would most likely require Loras' assistance which she knew her brother would be happy to give and she had no intention of stopping their relationship. Though she would advise them to be more vigilant and discrete.
This was all, however, after Renly discovered Margaerys knowledge. For now she could have a little fun at his expense. He would forgive her she was sure, if only because he would not want Loras to be upset.
She accepted the wine with thanks and raised the cup in return to his, "What better reason could there be?" she agreed with a nod, "Except perhaps that this is the first of many successes"