Unlike some in his former circle, Regulus does not believe Muggles are contagious or that you can lose your ability to do magic from getting too close to one. Although, admittedly, he had considered it a slight possibility before Mikoto's occasional lessons on genetics.

This means he doesn't pull away out of concern of 'muggle cooties'. Instead, he just hopes her hand doesn't get too close to where the Dark Mark was. Even though it had faded to invisibility, he knew it was still there. He had several theories on just what that meant.

There's a twitch more of a smile at the mention of Mikoto. "No, she'd never say that. But she might say that if I wasn't here she would have missed out on some interesting experiments in comparative magic."