Yes, every family is different. But, for parents to more or less tell their kids that they come second to the marriage is rather selfish, not to mention could be hurtful, depending on how the kid takes it.
While it isn't wise to make a child the center of the universe, so too is it not wise to make he or she feel like number 2. There is something very cold-hearted about any parent openly acknowledging that their kid is not first in their heart. Because lets not forget, a parent is the first love (and sometimes the only real love) a kid ever has. So, to them, the parent is number 1. Therefore, to them, they are also number 1 to the parent.
And, yes, you all can tell me how immature I sound, or that not all kids are how I describe. Nevertheless, ranking family members in order of importance is just about as crass as when guys give their girlfriends a numerical value, i.e. "My ex was about a 5 or 6, but baby, you're a perfect 10!" Making your marriage # 1 (or "first") and your children #2 (or "second") is just about as silly as that, and I'm glad some couples know better.
Please excuse typos/funny wording; I use speech-recognition that doesn't always recognize!