Q. Why was it considered adultery when David had sexual relations with Bathsheba (and rightly so ! 7th commandment ……….) and yet not when he had sexual relations with his 7-8 wives and many concubines. In reality he was ‘living’ in adultery all the time….I have struggled with this for most of my Christian life and have never found a satisfying answer. It is totally illogical to me.
There is a difference between adultery and polygamy. If a man who is married has sexual relations with a woman who is not his wife, that is adultery, whether or not the woman is married. And if a man has sexual relations with a woman who is married to another man, that is adultery, whether or not the first man is married. This, as you noted, is explicitly forbidden by the seventh of the Ten Commandments.
However, if a man has more than one wife and has sexual relations with each of his wives, that is not adultery, since the man is not having sexual relations with a woman who is not his wife or a woman who is the wife of another man. It is polygamy. Please see the post below for a discussion of whether polygamy is sin, according to the Bible. I hope this post will help answer your question.