So what she did wasn't okay but she didn't mislead him? So what did she do wrong?
How is his role worse when she held all the power and initiated the act knowing it would "cost" him much more should they get caught?
Saying "no" to a noble is not viewed as an option by many in their service because of the power they hold. The affair literally happened in the same episode where Alicent reluctantly slept with the king. They parallel wasn't coincidental. The whole point is to show how the Targaryens use and manipulate those around them.
Also there's no way he would have slept with her if he knew about Daemon. That would have been completely out of character because he would have realized she was just using him.