Question for the ladies (guys you can chime in as well), please help?

O.k let give ya some background info. My gf and I have been together for 2 years now. We live together, I'm planning on marrying her and I love her more than any other woman I've ever been with. We had a huge argument last night because over the last 4 or 5 months now she's been gaining a lot of weight (not pregnant). Now before you say I'm "being superficial", or the cliche "it shouldn't matter if you really love her", its not a deal breaker, and I never have and never will cheat on her. But it's been bothering me for quite some time and I've been wanting to talk to her about it forever, but didn't want to hurt her feelings and start the inevitable argument that we wound up having. Of course physical attraction isn't the MOST important part of a relationship, but it IS important. I've found myself not being as attracted to her, and that scares me because if I feel this way now, what happens 10 or 15 years from now when we're married with children. She has tried some exercises in the past, and I've done them with her for motivation. I even bough her a Wii Fit video game console (her idea even before the weight), to make working out more fun. But she gets discouraged and feels like it doesn't help. But she really doesn't give it much time. She of course said I was being superficial, and that it shouldn't be as big a deal as I'm making it out to be. But my question is, is it o.k for your guy to express his unhappiness about your weight in A RESPECTABLE manner, if he sees that you aren't making any attempts to address it on your own, and it really has been bothering him?