Why Adultery is legal and morally harmful?
Adultery is an immoral act against The 10 Commandments. It's also illegal because the civil marriage contract binds both parties to be faithful in the bonds of marriage. Having an extra marital relationship is a starting point that breaks the bonds of marriage.
Jesus Christ says that if a man or woman looks at another person outside the marriage sanctity, he or she has already committed adultery even without performing the actual sexual or emotional acts of love making. I admit that I was once carried over by the culture of sexual exploits and conquests that disrespected the marriage sanctity just to gain sexual lust that lead to destructive acts. Realizing the meaning of self control and respect of the marriage bond of oneself and of other parties marriage bonds preserve the foundations of marriage. "What God has put together in marriage, let no person break or take away or put asunder". True, to err is human, to forgive Divine. One need to be remorseful and avoid the same immoral acts. It's easier said than done especially when sexual acts are so exciting and adventurous, when many adults are consenting, when there is a culture of double standards and hypocrasies, when many people and some traditions allow those with wealth to buy consenting girls or women for financial supports, when polygamy is socially accepted; the individual usually men are left to decide on the advantages of faithful one marriage bond. The advantages are legal and moral obedience bring strong healthy family members and truthfulness to the love in the marriage bond.
"Because of the hardness of their hearts", Moses allows divorse or annulments in cases of irreconciliable differences, insanity, or voidable marriage contracts from the start when one or both parties is or are lieing and not forthright with his or her previous marriage contract with other parties.
There is a saying in Filipino culture that no person can paddle his boat or canoe in two opposing waters running in opposite directions. The boatman's boat is usually capsized and is in great danger of drowning due to the strong water currents. The strong currents of sexual acts for other girls or for other women that the unfaithful husband experience will break the marriage sanctity and respect, thus, becoming unproductive and destructive. Death after trying to have multiple orgasms, the unfaithful husband or wife die with heart attacks at the overpowering lustful bed is another Filipino sayings.
The lesson of faithfulness in the marriage contract preservers the sanctity and respect for one another in the family that procreates and raises responsible and productive children and parents. If one commits adultery, one needs to repent and tries his or her best to avoid the same mistakes. Many times, it takes humility and courage to ask forgiveness and to have inner strength by continous reliance on the Divine Graces of Jesus Christ for freedom that overcomes one's slavery to sexual lust and poor judgements.