We’ve always had them. Since we were old enough to go to a bar more often than study hall, we’ve had beer guts. It’s just that before we had kids, we could suck them in.
Let’s do a demonstration .. Stand up.. and .. suck in. Great. No gut. No problem.
Now, go and grab a niece or nephew or something between 20 and 40lbs and hold it on your hip. Now try to suck it in. You can’t. To balance that extra weight, you need to push your extra weight out. It becomes extra support for that crazy little son or daughter.
Now, if you’re a dad like Brad Pitt, and start fatherhood with a spectacularly chiseled physique, while your beer gut will exist, it won’t punch out as far as .. say .. mine.