Is seeking refuge from the whispers of Satan that call to polytheism and immorality—such as lesbianism—obligatory, and is ignoring these whispers considered polytheism, and how can one ensure the soundness of their faith and their innocence from polytheism when exposed to these whispers?

You must completely disregard obsessive thoughts, avert your gaze from images of sinners and nudes, and act according to the fatwa of Ibn Hajar al-Haytami regarding obsessive thoughts. The sufficient cure for them is to completely ignore them. If one pays no attention to them, they will disappear after a short time. But if one gives ear to them, they will increase until they drive one to the brink of madness. Whoever is afflicted with obsessive thoughts should seek refuge in Allah and stop.