> You've found your mate, so less incentive to go to the gym / stay in shape. You've just had a kid, so less time overall to do so, and (especially!) less sleep in general, which causes all sorts of problems.

I mean if you met your spouse at the gym its probably a reason to keep going. It's a common activity you both enjoyed when single so you still enjoy it as a couple.

Running after kids should keep in you in shape as well. Although I guess you could just watch TV with them all day but that's not dependent on having a kid.