We don't see how they cope with the realities of life. We don't see how they adjust to living together or what happens after the wedding and the honeymoon. Most of all, we don't see how couples stay together, year after year, through raising kids, job losses, ill health, in laws and all the other daily things that make up a life.
You're not just getting married and perhaps gaining some new children in the process or bringing a new spouse into your life -- your children are gaining a whole new family through your marriage. Making them feel like they're part of the decision-making process is a great way to start things off on the right foot.