"Blood is thicker than water." I have been considering this perspective in light of the Biblical record. While family does play an important role in the development of the individual, it is not the closest relationship a person can have. "For this reason a man shall leave his father and his mother, and be joined to his wife; and they shall become one flesh" (Gen. 2:24). What is this saying? First of all, it is a commentary on the relationship between a man and a woman inspired by the relationship between Adam and Eve. A couple becomes united in marriage, and that relationship predominates over all others.
If either of the couple's family intrudes into this relationship, the marriage is threatened. It stunts the solidifying of the bond. In short, it is sin. Blood, most decidedly, is not thicker than water. When a man marries a woman, there is a change of order. His wife becomes more important in his decisions and life than his own family, because his wife is one with him. This goes vice-versa.
Without "leaving", the marriage is doomed to enmeshment (with the family) and underdevelopment (in the marriage union). One of the members of the marriage is bound to feel hurt, resentment, and eventually, bitterness over their spouse's emotional infidelity. This explains the wide variety of bitter jokes men make about their mothers-in-law. These men have been deeply hurt by their wives' emotional infidelity to keep personal information and decisions in the marriage, so they lash out in bitter and sarcastic humor.
Men are capable of this kind of infidelity, as well. A man may be close to his mother because she enable him by doing his laundry and making him meals while he his living "on his own". His inability to leave his parents' care continues his childhood. If this pours into a marriage, the wife becomes bitter and resentful. She sees the mother (or father, or both) as competition.
Eventually, the very real emotional infidelity drive the spouse away, and the first sign of this is "clamming up". In other words, they refuse to share personal information out of fear of it being broacast to the in-laws. The next stage might be avoidance, where the spouse intentionally does not come home because of the pain of seeing an unfaithful husband or wife. The third stage is fulfillment of need. If the husband or wife cannot trust their spouse, they will create one. This could be in the form of a co-worker or friend. In any language, this spells danger. An adulterous affair results, in this case, out of the emotional adultery of the spouse.
God has our best interests in mind. He protects marriage by outlining its parameters (one man and one woman) and creating the bond that strenthens it (in addition to sex, emotional transparency and the preservation of trust). Old Testament punishments for adultery were severe, and we should not think that it is any less severe for emotional infidelity.
Christ states, "So they are no longer two, but one flesh. What therefore God has joined together, let no man separate." (Matt. 19:6). The word "man" is actually anthropas, meaning, human.
For those who may read this article feeling sorrow for their sin against their spouse, there is hope. Christ came to swallow up the whole of your sin in his body broken on the tree. He has suffered the brutal punishment for your sin, and has forgiven you for all of it. If it is time to make ammends with your spouse, do not wait another minute; your marriage hangs in the balance. Blood is not thicker than water.