To me, "leading man" in dance relates to lead/follow in ballroom. The leader provides the frame for the follow. It's almost like the idea of a spotlight on a beautiful piece of art. Your eye is on the art, but you are also aware of the spotlight, because without the spotlight, you wouldn't be able to see the beauty of the art. The art is the follow; the spotlight is the leader.
ETA: In my analogy, the art and spotlight are stationary -- just realized if you think of a spotlight following a figure skater in a gala, this could get confusing.
It's just that the leader is doing its best to present the follow in the best light. When this happens, you watch the follow, but you are also aware of the leader because you can see what it does to make the follow look so good. That's why you essentially watch both of them.
ETA: In my analogy, the art and spotlight are stationary -- just realized if you think of a spotlight following a figure skater in a gala, this could get confusing.
