When I was single and renting I had tons of free time to workout, swim, bike, rollerblade, etc. I ate what I wanted to and when so it was easy to control. I was about #195, 6’1" and in great shape at age 30.
Then I met “her” and my free time got sucked away. Now when I had dinner it had to be what “we” wanted to eat. Gym time got smaller and smaller. I essentially gave up after we married and was at #240 at age 34.
I hated being that big and changed my ways and got back down to #200 by age 35.
My son was born last summer which again sucked all my free time away and dinners became takeout food. I’m back at #210 at age 37.
I was in better shape when I was younger. I’m still single and looking.
Let’s see…I see bits of Kalhoun, GilaB, **Tahssa **and **Phlosphr **in my reasons:
I’m older now. I’ve steadily and slowly gained weight over the years, so any random event X in previous years saw me thinner and in better shape than now.
I feed my family, and do that Mom thing of nibbling at their leavings even when I’m not really hungry, and waiting for my godsdamned slow husband to finish gives me a lot more time to eat more food I don’t need.
I stopped smoking. That was a killer for the metabolism.
I got depressed, pregnant and then depressed again. I’m not, unfortunately, the kind of person who can’t eat when depressed. I put on more weight post-pregnancy than I did while pregnant, worried about the preemie and eating crap hospital food for four months.
I went from working at a fast-paced doctor’s-type office where I was walking or running all day long to…well…sitting on my ass in front of the Dope. Yes, I have a two year old, but she’s not one of those crazy run-around two year olds.
We’re fairly poor right now, and that means I’m buying and eating a lot more filling starchy refined carbs. I tend to save the “good” food (fresh veggies, protein) for the kids, since their bodies need the extra nutrients to grow.
My wife actually weighs less now then when she was in High School, one of the few rare people I know who can claim this. I alas can not claim this factoid. But I was also rail thin in high school, so it was to be expected.
Overall though I and my wife are in the best shape of our lives to be honest. We work out three times a week and watch what we eat. Since the first of this year I am down about 17 pounds and she is down over 20 pounds. Doing the BMI thing (adjusted for age mind you-- Account Suspended )
puts both of us at our current weight in the top 25% for our age group. So we are pretty happy about that.
For us it really was the dual aspect of increasing our exercise and really watching our portions. Cutting back on snacking was a big factor too. It is just depressing as you age isn’t it! 
But we want to live long healthy lives and the only way to do it is to get that weight off and keep your body up!
It’s the effects of ageing, IMHO.
I am now 43, have been single for the last 14 years (since age 29), and I still gained weight in my early thirties. I think the weight gain was due to having a steady paycheque again after a go at returning to school in 1992-3. I ate more.
OTOH, I am actively trying to get into better shape now, because I can now feel those effects of ageing.
Hell, no. I was overweight when I was single. I married and gained more weight. Now, I’m the lightest and fittest I have ever been in my entire life (and still married).
Ditto. When I was single and dating the husband-to-be, I was flabby and weighed 15-20 pounds more than I do now. I was tallish and could carry it off.
OTOH, 25 years later, the husband, who had been quite skinny, and now is about 20 pounds more than when I met him.
This is the phenom that GilaB mentioned. I’m astonished; in my experience men tend to be the speedy eaters, wolfing and scarfing. Apparently I’m mistaken!
One of the forces which caused me to put on weight while married was the fact that my wife considered food a “love language,” just like my mother before her. When either of them cooked, it would hurt their feelings if I declined to eat their offering. It took me years to negotiate a strategy that gave them an outlet for their instincts without turning me into a dumpling.
When I’m single, I try to stay in shape for “the prowl,” but when I’ve had boyfriends the incentive was still there, just different. I liked to stay fit then because I enjoyed the idea that they took as much pleasure from it as I did. 
I was also stick thin when I was young and single. My getting married coincided with my service in the military, so I was bulked up and much more fit then, but then I broke my back doing everyday things are now chores in and of themselves, so I don’t get the exercise I’d like.
By the way, Ginger, I saw your photo in the Doper pic thread and truly thought, “Wow, Ginger’s looking good these days. Is she still married?”
Well done.
Part of it is being on the prowl. I would say a larger part has to do with the fact that I have more free time on my hands when I’m single. I get bored and go to the gym.
I was about the same. I don’t really lose/gain weight for long, I go back and forth.
When I was single I was in great shape because I had the time to devote to working out. It wasn’t being married that killed my figure as much as graduating from college and getting a full-time job.
[hijack] When I’m single and am looking for a female, I don’t really think of my search as a “prowl”. [/hijack]
Oh, and to answer the question. What msmith said.
What do you think of that search as?
I’ve lost 30-something pounds since I broke up with my ex, so… yeah. Granted, it began as just stress, but once I started losing weight, I started WORKING at losing weight. And while obviously *most * of the reason I work at it is because I just physically feel better when I’m thinner and stronger, I have no problem copping to being shallow and vain, either. And afraid. Because at some point, I’m going to have to get naked in front of someone new for the first time in nearly a decade! :eek:
I agree with this. Think about it, don’t you now-partnered types have meals or snacks with the spouse just to keep spouse company? And maybe eat even though you weren’t planning to? That way lies lardiness.
I was certainly thinner when I was single, but I think I’m in better shape overall now—in my late thirties—as I generally eat better, go to the gym regularly, and don’t indulge in the chemicals I did in my youth (well, most of them anyway).
However, a few years into marriage I did find myself embarrassingly inactive, doughy, overweight by a good 20-30 pounds, and looking a lot older than I felt—the stark realization of which (mirrors can be quite cruel sometimes) helped prompt my return to the gym. Needless to say, it’s been a lot harder and more time-consuming getting back into shape than it was getting out of it.
Some people are “nesters” when they are in a relationship. They prefer to snuggle on the couch after a good meal rather than go out, whereas when they are sinlge they are more active, going out to socialize and such.
I found a got a little bit more fit after I met my fiancee, but only because we are both active, hobbiest athletes, and we ended up taking on a couple of each other’s activites too. She is a die-hard cyclist. It’s not my passion, but I got a new bike to keep up with her. And she’s taken on one of my sports.
Oh, and PunditLisa is right about work/ fitness level. Full-time job was far worse on my waistline than a relationship. (Now I work out a lot more than I did my first year on the job though.)
I think it has more to do with age than marital status. When your young it’s easy to lose 5-10 lbs. As you get older, not so easy. I have to work just to maintain my weight. My husband probably weighs the same as he did in high school. It’s moved around a little but he never gains weight.