That's perceiving. If you DODGE the bullet then that's a reaction. If you stand still and just look and take it it's not. For example, I have gun and I'm about to shoot you with it, you can see the bullet coming, but you can't dodge it. That means you're TOO slow to dodge it. Meaning, you did NOT react in time to dodge it. In luffy's case you have no way to prove that he would've dodged it, not to mention that he wasn't even looking at it. If he reacted, he would've dodged it, and no he didn't, it hit him. I sent you the literal definiton of what a reaction is, where is the response? Luffy isn't responding, not to mention that even if he looked at the beam, that isn't considered reaction, it's just him perceiving what's coming at him.