Because he had wanton and depraved eyes on. She wanted a double serving of wang, and reasoned - not illogically - that her penislust would be shared by her brother and husband. Since she batted not an eyelid at the idea of incest with her gay brother, it is reasonable to assume that they'd already experimented with a pratical comparison of various rusty wagon wheels. That is, she and Loras had already knocked boots together, and she had the look of a lascious slattern which scared him. He was not ready for a threesome involving siblings.