I feel like a broken record, but I will keep saying it over and over again: not all fat people are fat due to bad eating habits.
To the OP, a couple of questions:
1. As another mom-type, I'm with UpsandDowns. What's the rush to get married? I dated my husband for 7 years, on and off, before we got married. We've been married for 24 years, and it's still challenging. Don't rush.
2. Have you discussed your feelings about fat with her? Do you know hers?
3. If she never gained a pound, would you be happy with her for the rest of your life?
I think they'd all be worth asking yourself before you proceed.

Whatever you decide, I hope it works out wonderfully and that you're very happy.