"The term “one flesh” means that just as our bodies are one whole entity and cannot be divided into pieces and still be a whole, so God intended it to be with the marriage relationship. There are no longer two entities (two individuals), but now there is one entity (a married couple). There are a number of aspects to this new union.
As far as emotional attachments are concerned, the new unit takes precedence over all previous and future relationships (Genesis 2:24). Some marriage partners continue to place greater weight upon ties with parents than with the new partner. This is a recipe for disaster in the marriage and is a perversion of God’s original intention of “leaving and cleaving.” A similar problem can develop when a spouse begins to draw closer to a child to meet emotional needs rather than to his or her partner.
Emotionally, spiritually, intellectually, financially, and in every other way, the couple is to become one."