YANBU, but motorway driving is generally really easy driving (joining in the fast lane excepted...).
You will probably find that once you've been on a couple of times, you'll be fine.
An advance lesson would be a good idea.
If you're not fine with it, sobeit. It's not the end of the world. Depending on your lifestyle it could be restrictive, but you've got this far without doing it, so I guess you'll manage.
Although I've been driving for about 15 years, I don't understand this notion that you're not capable if you don't drive (or don't drive on motorways in your case).
Now, if you were to say you refuse to turn right in your car....
(oh, and get DH to stick to his promise - you don't need to be driving round in a big tank!)