As a new dad who has put back on some weight, I'd say the biggest reason is not about "keeping my partner" but purely how I rate different things I need to spend my effort, time and energy on. Less time and more stress makes me more likely to make some poor health decision - for example heading out for a walk or a run would help but right now
I'm knackered and just want to sit down because it's the first break I've had today. Or order in food because my plan to cook earlier failed due to an unexpected _something_ or just plain comfort eating.
Kids are great, but undeniably add time and stress. Maintaining good healthy habits is not impossible, but it's easier to let them slide.