So whenever you have a group of women together, eventually the topic of weight will probably come up. Often women unhappy with their weight will try to make themselves feel better by saying, "Well, I've gained a lot of weight, but my husband doesn't mind. He loves my fuller figure!" I never feel that I can argue with this, because 1. I hate talking about weight and 2. it's mean, but it really annoys me. Of course your husband minds!!! You are way less attractive now!!
Let's be real (since it's the Internet and everything): for every couple, it's important that the two people be of similar attractiveness level. If they are not, things are miserably unhappy. Attractiveness is about more than just weight of course (for men, it usually includes things like income, employment status, their level of assertiveness in daily life; for women, it includes grooming, enthusiasm for sex, kindness and household skills; for both, intelligence, reliability, "being fun", etc.). But weight is probably the biggest determinant of physical attractiveness, given that most modern people are clean, fairly well-groomed and disease free. And for women, most (75%?) of attractiveness means physical attractiveness.
So if two people get married, and afterwards one gains a lot of weight, that means the attractiveness levels are now out of wack (since they used to be equal, but then one declined precipitously). And if it's the woman who's gotten fat, then these levels are really unbalanced. Sometimes people in this situation eventually get divorced: they almost never would admit to the real reason, but that's what it is.
Other times they stay miserably married. I remember one couple I used to know (B worked with the husband). The wife had two kids and got really fat (like hundreds of pounds overweight). The husband had a constant look of misery whenever the two of them were together, I think partly because of the social embarrassment that she represented (since he moved in circles where fatness was very rare and not at all socially accepted--and your spouse's appearance, especially for men, does serve as a reflection of your social dominance). She tried to compensate with endlessly placating behavior, but of course it didn't help. It was painful to watch.
Probably the best case scenario is for the husband to lose attractiveness too, so they once again are correctly matched. (I have a cousin whose husband did this, so now they are both obese; as a man, though, he had to do some other things as well like be marginally employed, etc.).
Or everyone could just maintain their attractiveness as best as they can (which means a lifetime of watching your weight, but it's better for you anyway). As you both age, you will get less attractive; but at least it will be in sync (one reason very large age gaps are problematic).
NB: This whole post is about gaining weight once you are in the relationship: if you were fat to start with, it's a completely different scenario (since then your attractiveness level hasn't really changed). In fact, if you started out fat, and then lost a lot of weight, this also tends to be destabilizing for the relationship (the divorce rate for gastric bypass patients is perhaps 80-85% post-surgery).