There was a study carried out a while back by Newcastle University that found that living together resulted in the male partner becoming healthier and the female partner becoming fatter. This is obviously not going to be true for all couples, but when you think about it, it does make some sense.
When women live on their own or with their friends or parents, they are basically preparing meals for themselves. So they can easily control exactly what they are eating, and can easily ensure that they are not eating excessive amounts of food every day. They may also have a strict food budget which restricts them from buying too much junk food every week.
However when they start living with a partner things start to change. All of a sudden they have a bigger budget to spend on food, which means junk food may be more readily available.
It also means that in a lot of cases the women are preparing meals for the both of them, and over time it is often the man who dictates what kind of food they eat. Therefore because men usually tend to like big hearty meals that often include plenty of meat, the female partner will also prepare bigger meals for herself as well, which obviously increases the chances of her putting on weight.
As regards the male partner becoming healthier after living with a partner, it is a little harder to understand the logic. This is because you would assume that single men often go to the gym and look after their body more in order to attract a partner.
However I can understand the reasoning to some extent because men often live on their own before moving in with a partner, and a lot of men eat a lot of junk food because it is quicker and a lot less hassle. After all you don't really want to prepare a big roast dinner when you are just cooking for yourself.
It is also logical that women encourage the male partner to eat healthier when they start living together because women in particular do not want to put on weight. However there is often a compromise because with healthier meals come bigger meatier meals in a lot of cases, as I've already mentioned, which would explain some of these findings.
Of course these studies are carried out all the time, and you can never read too much into them. However they are quite interesting, and I can certainly understand why men and women often see their weight change after they start living with a partner.