only do it if me and the missus ......? wear only grey clothes, actually they said mid grey 18% reflection, what does that mean?

Means they are quacks and do not know how to take good pictures. A good photographer can take a picture under almost any circumstance. Find a different photographer.

it means they are probably confusing light measurement with doing a professional and job and failing to understand that as a wedding photographer their job is to portray your very special day, get somebody else and make sure you see some of their work first

Go ahead and do it. It will be easy to fix the images in Photoshop. Duplicate the original into a new layer. In one, set the white point on her gown and in the other, set the black point in her tux. Merge the two and - voila! Perfect wedding pictures.

OMG NO!!! Look for a new wedding photographer! Who the heck wears grey on their wedding day? Is this photographer a professional? If so they are supposed to know how to adapt and make the most of the surroundings, clothing, etc..

I would get a different photographer.