I put on about 40lbs after I got married
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This was the strangest thing. I actually ate less when I was married because I was so broke. Yet I gained weight! It was as though my body said, "I got myself a wife - now I can make myself look like complete garbage since I'm not in the market any longer."
My boyfriend gained a few pounds as soon as we started dating, because he was always finishing my plates for me (he's been raised to leave nothing on the plate, and his parents gave me HUGE amounts of food to make sure I ate enough, but I'm a light eater... So he finished my plate all the time). Also, because he stopped going to the gym when we started dating, so that we could spend time together (between school, work, gaming and the gym, he decided that something had to go haha).
So there can be a whole lot of reasons why! He's lost a few pounds since we started eating healthier, too.