I wouldn't be concerned about shooting the ceremony and reception with flash. If you don't know how to use flash well enough to shoot a ceremony and reception with flash then you probably shouldn't be shooting weddings (that sounded harsh. I didn't mean it to!). I would be concerned that if they don't do a first look all of their portraits will be with flash. If I meet with a couple whose ceremony will be after it is dark I won't book their wedding unless they agree to do a first look. Why? Why would they want to pay me a lot of money to not have their pictures look like what they see in my portfolio? Some photographers can do amazing things with flash. I'm not one of them and I know that. I love me some natural light!

I would explain to them that if they don't do a first look then their pictures aren't going to look like what you've shown them. Educate them to know exactly what a first look is. Some couples don't want to do one because they want to have that moment where they see each other walking down the aisle. But, explain to them that they can still have this moment during their first look.

If they still don't want to do a first look then it's not your problem :) Maybe they will book you for a day after session!