Thinking about committing a forbidden act

The answer – and Allah is the One who grants success and assistance – is that what has been mentioned in these two questions is among those things that a Muslim ought to rise above and avoid. This is because of what has come in the ḥadīth: “The believers are those who come to a halt at doubtful matters,” and: “The lawful is clear and the unlawful is clear, and between them are doubtful matters…” and so on, or as he (May Allah bless him and his family and grant them peace) said. What has been mentioned is among such doubtful matters, as is indicated by the fact that Allah, Exalted, has forbidden in His Book and upon the tongue of His Messenger, May Allah bless him and his family and grant them peace, the causes that lead to falling into the sin of fornication. He has forbidden being alone with an unrelated woman, and commanded lowering the gaze, and commanded seeking permission when entering houses, and commanded women to cover their shame with silence and to cover their nakedness by remaining in their homes. And he, May Allah bless him and his family and grant them peace, has forbidden following one glance with another, and he has forbidden them from displaying their adornment – even to the extent of forbidding them to stamp their feet in such a way that it should be known what they are concealing of their adornment.
Then Allah, Exalted, has forbidden listening to singing for various interests, among them that it stirs and arouses desire – to the point that they said: A chaste woman has never been enticed away from her chastity with anything like singing, and so on. What we are dealing with here is close to that; and the fact that doing what is mentioned in the question is for a purpose connected to the wife does not justify it.
Thus, the point of what we have mentioned is that in this issue there is a doubtful matter which it is fitting for the believer to avoid and rise above.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2