Having kids can change the dynamics of your marriage for sure.  This article explores the soul mate aspects of marriage.

Christianity Today offers an insightful look into what it means to be soul mates.  Regardless of your spiritual beliefs, this article explores the nature of soul mates, and talks about the work involved in maintaining this status with your partner.  There is some godly talk as well, but I think the point can be appreciated whether you share in this belief system or not.