I don't think that our definitions of adultery are similar, because yours allows the concept of consent to take place, and also places the "ownership" of the marriage with the people involved in the marriage.
I believe that God joins the married couple, and as such, the marriage is before him, and anyone who tries to break that joining, even if it is the two married people themselves, are guilty of adultery. The consent to do something like that is God's to give.
A husband cannot give consent for his wife to sleep with another man, because that consent is not his to give. That is why, if a man divorces his wife, for any reason other than sleeping around outside of their relationship, he causes her to commit adultery.
Matthew 5:32
But I tell you that anyone who divorces his wife, except for marital unfaithfulness, causes her to become an adulteress, and anyone who marries the divorced woman commits adultery.
The exception there is because if the wife committed any sexual immorality on her own, she was already an adultress.
Again not true.
The marriage act predates the fall, and God's punishment of making the woman inferior to the man. When God joined Adam and Eve, they had not sinned, and were not the property of one another. That is how God
intended it to be.
While women might have been property in OT times (and even in parts of the world today), that does not justify the consent argument. You cannot use the traditions of man to justify what God has put in place.
As God instituted marriage, there is no right for anyone to loan out his wife to another. This is sin.
You assume that the husband is the only one who can find the wife in this position. It can be anyone, doesn't necessarily have to be the husband. If the husband gave the consent, and the wife did the act, and the neighbor caught the wife and the lover, they would still be stoned.
Big hole in your argument, my friend.
Your definition of sin is wrong.
If there is no harm done to people, but you do something that is contrary to the standard of God, that is still sin.
A man can give consent to his wife, to sleep with someone else, and she can do so, and they are all happy, but it is still sin, because God does not allow that.
Sin has nothing to do with the harm inflicted on people (although most sin does inflict harm on someone). Sin is missing the mark God sets for us, not reaching the standard of perfection that God is, and holds us to.
Sin is against God, primarily. Sometimes, it is against people too.
