Ron's presence was more of a kick in the shins for Luna, to get moving. In one fell swoop, she finds that a) Ginny knows that Luna "likes" her and b) Ginny's brother, who Luna knows can be a bit mean (as she said in whatever book that was) is supportive of her. There's nothing holding her back anymore.

Ron's not blowing up was more because it wasn't Ginny that Michael was protecting, true, and being a bit surprised by the situation. As I said, Ron hadn't been confronted with it before (and most teenage boys' lesbian fantiazies are more "two girls who are hot for me" not "two girls who are hot for each other and not me at all"... at least I think they are. Never been a teenage boy myself.)