"And the Lord was sorry that He had made man on the earth, and He was grieved in His heart," (Gen. 6:6, NASB).
The above verse is not a problem for Classical Christian Theism, nor is it a proof text for Open Theism. The verse simply tells us that the Lord was grieved and had sorrow in His heart for making man. Why? Because mankind had fallen into great sin, and this grieved the Lord. Does it mean that God didn't know mankind would fall and become sinful? Of course not. Cannot God know that they would become sinners and also be grieved when it happens? Of course. Let me illustrate.
I have children. I love them and provide for them, but they have grieved me in their various sins--as any child will do to his parents. I knew they would grieve me when they were born because I know they are sinners by nature. This doesn't mean I was surprised and didn't know they would rebel when it happened. Quite the contrary--knowing they would sin didn't mean I wasn't grieved when their rebellion and sin was finally manifested.
The open theist would have us believe that God was grieved because He was surprised or didn't know the depths of sin to which the world would fall. But surely--even in Open Theism--God knew that people would sin. So, this verse can't be claimed to demonstrate that God didn't know the future choices of people.