Margaery found it incredibly interesting that Renly was still so insistent on denying the truth, even knowing that Loras had confided in her. From her point of view it suggested he really didn't trust her which meant he was more savvy than she had originally given him credit for. She would need to earn his trust as he would hers but at least they were starting on a level playing field.
"I'm accusing you of nothing" Margaery informed him, "An accusation would imply that I believe you guilty of some sort of wrongdoing. Loving my brother is not wrong. No matter what anybody else would have you believe" she said her voice strong and unyielding.
She took a breath and offered him a small smile, "I am offering my understanding and friendship that is all"
Despite knowing that this must be difficult to process and may indeed be causing some confusion Margaery had to admit to running out of patience with these denials. She did not want to lose her temper, it was very rare she did and she also needed to remember that while she may be Queen, Renly was still the King and may not be tolerant with too many home truths.