Often it happens. I had a big health crisis a few years ago. It made me realize I had to lose weight to live - despite my husband at the time being a big FA. The more weight I lost - the more he tried to undermine my efforts. He would never admit it - but he kept cooking me cakes and offering me ice cream! It finally got the point where I realized he was going to keep undermining my efforts to the point where it could kill me. The more weight I lost - the more he lost interest in me in the bedroom and finally even really talking to me. It's a shame when a man loves a woman for her looks being thin and then stops loving her when she gains weight - and the reverse is just as sad - and sadly does happen.
If you love them you accept them as a whole, even if the whole gets smaller. The biggest problem I've seen and experienced is that sometimes the personality changes, they decide to change other aspects of their life including partners. So if major life changes begin to occur, it can be tough, really it depends on how much of that woman you fell in love with is still around.