I can say that Marley is not a huge fan of rain either, and does hesitate, but he will then go out.
What helps me is a really big umbrella so he doesn't get that wet. When I don't use it, he does just ok, but with it, he will stay out long enough to do ALL of his business instead of running to pee and bolting for the door.
Get a big umbrella /forums/images/%%GRAEMLIN_URL%%/smile.gif