Not sure what you're trying to say. We already know that adultery and murder are sinful and God punished David for both of these, though he was shown mercy in that he wasn't put to death. On the other hand, there is an abundance of scriptural evidence that God approves of polygyny, men having numerous wives. The very conversation of Nathan with David confirms that, let alone all the heroes of the faith like Moses, Abraham, Jacob, Gideon, Caleb and others who were polygamists. If God had a problem with it , He would have said so, rather than merely regulate the practice. If it was sinful for David to marry more than one woman, do you think he would commit a "sin" by marrying Bathsheba so soon after God had exposed and judged him for what he did to Uriah? The Talmud taught that their relationship was ordained by God and that David regarded Bathsheba as the wife God had ordained for him.
Bathsheba wasn't raped. She willingly agreed to adultery without a complaint that we read about. I suspect that she was already in love with David before any of this came about, and she might have deliberately allowed David to see her bathing. David didn't sin by looking at her and desiring her sexually at the outset. The Bible doesn't teach that. But, once he knew the woman was married, lusting for her was wrong, as Moses taught and Jesus confirmed.
At some point in their relationship, David promised her that her son would be king of Israel. Why would he promise her that when he already had older sons by previous wives? We don't know if that was what she asked in exchange for him having sex with her, but I think its possible. So, personally, I don't believe that Bathsheba felt like she was forced to have sex with David. There's no evidence that she was afraid of him. What was likely forced upon Bathsheba was her marriage to Uriah who was likely old enough to be her father. He was a Hittite so this may have offended her was well, since all Jewish girls hoped to give birth to the Messiah. He was also a warrior and might not have been that gracious in the bedoroom. So, I don't think she was happy in her marriage to him and she sensed that she had a call to greatness as well.
I don't believe it was any accident that both of Jesus' earthly parents were descendants of David, since He was predicted to be the "son of David". The fact that two of Bathsheba's sons would be chosen by God to be ancestors to His Son speaks volumes about the nobility and virtue of Bathsheba, David's eighth wife. I really believe God had ordained their marriage, they sensed it, but got ahead of God. There is also this little known fact that Israel's soldiers gave their wives temporary divorces when they went to battle, so that they could remarry if they were missing in action or killed. Bathsheba was technically unmarried, and sex was regarded in those days as a means to create a marriage. So, the king and Bathsheba were probably justifying their affair that way.