It doesn't take much for someone to sacrifice their health for something – whether it's for a job, leisurely activities, stress eating, or just not caring.
Marriage adds more reasons on top of this. 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.
Can anyone recall a "marriage makes you healthier" trope? Seems like a straw-man to me?