Robbie, I think we may all have thought a bit the way you're thinking at times. I really feel for you. EDSS 7 is hard - I have been 9 during one relapse for 2 weeks and couldn't hack it at all - and it is hard for you and your wife, but thinking like this will drag you down.
Even wheelchair users can dance you know! I have seen some of my colleagues dance using their wheelchairs, great fun, there are ways around everything.
Everyone brings something to a relationship, no matter how little you feel you have to offer right now. I grew up with a Dad who was seriously mentally ill, still is, and I have to say that that was a situation where he should have left for the sake of the family and got some proper hellp. In your case, your body has changed, it's not doing what you want and need it to do, but you can still be 100% there for your wife and your relationship can be good....it's not dependent on legs that dance...........
Hope things are looking more hopeful soon, and keep talking Robbie.