I don't see how Luke could react at all given the spoilers we have. He's surrounded by patty, babette, and Rory, and maybe some others. It's not like he's going to storm out, nor will he just run up to the stage and tell her loves her too. If anything, they'll probably spend the rest of the night/party, until one of them decides to leave, not talking, both of them still processing what just happened and what it means.

And since Luke's upset when he hears that Lorelai didn't mean it, he can't have reacted too poorly. It seems more likely that he believed what she was singing than not.