I read that pregnancy (for a woman) chemically and permanently changes the body so that it is harder to lose weight post-pregnancy. That is why I'd be okay with never having children lol. I'll probably have kids if I can though - just one or two.
My mom started to gain weight she couldn't lose (without exercising) after her 3rd child, so 2 kids would be my cutoff. I hate exercising myself but I'm 27, 5'2.5", and I've been around 105lbs for over a decade, and I still get mistaken for a teenager (look young)...I pretty much eat what I want without gaining weight, but I do realize that I just don't even eat a lot compared to most Americans, I don't drink a lot of coffee/soda/alcohol (basically water or powdered mix drinks are my go-to), I turn away desserts if I'm just not feeling it, I eat until I'm satisfied but not belly-aching full (unless it's a special occasion, like Thanksgiving or a celebration), so in a way I'm always engaging in portion control. But pregnancy and the hormones/chemical changes therein would probably be a game-changer for me in terms of balancing/encountering weight gain for the first time in my life, and trying not to keep that weight down post-pregnancy.
So for a lot of skinny women who gain wait after pregnancy, that specific type weight gain is probably something they've never encountered before. Men don't have to go through with this because their bodies aren't the ones popping out the babies, so I think as a man, if you decide to re-marry and have biological kids, if your wife starts to gain weight, be sure to be on top of her post-natal regime of getting her back into shape and plan ahead. Don't rely on her to lose the weight all by herself, you need to encourage her to diet/exercise and give her new tools she needs to help her tackle her sudden weight gain, and keep her body looking hot.
Otherwise if you want a thin wife AND kids you should probably find someone who is already engaging in a fitness exercise/lifestyle or someone who just has good genes with managing post-pregnancy weight.