One of the reasons why women gain weight after marriage is that most women get married very young. Most people gain more weight as they age. When a woman is 30 y.o. she rarely would weight as much when she was 19 whether she is single or married. Gaining weight with age is a part of life and shouldn't be met with disgust and surprise. Most women get married very young comparing to men, so don't be surprised if your wife is a few pounds heavier than when you met her.
Another reason for weight gain is having kids, many women want to have 2-3 children and it puts additional stress on a woman's body. Some women don't even have time to loose weight after a first child when they get pregnant with a second.
It's alright for a woman to gain some weight after marriage, I think. Most men probably wouldn't even notice if a woman gained about 10 pounds. The only problem arises when weight is extreme and causes physical and psychological affects on men and women.
Also, when women get married, they have more responsibilities, stress, which affects weight gain too. Most women have to deal with raising children, going to work, have other issues in their lives. If you really want to help your wife to loose weight, provide for her stress free environment, so she will have time to take care of herself.
Here are a few tips for you to help your wife to loose weight and reduce stress: take care of all financial responsibilities in your family, let her go to work if she wants to and joys her job, hire her a nanny to help her with kids because raising kids is a difficult and stressful job for any woman. This way, your wife will have more time and energy to take care of her weight and emotional issues of overeating.
If you can't afford then don't complain that your wife is heavier than she was before. Love her the way she is and maybe your love toward her will encourage her to look good for you at all times.