Ibn Taimiyyah, may Allaah have mercy on him, said: “The effects that lust has on a person, such as loss of intellect and knowledge, corruption of manners and religion, and being busied by it leading to neglect of religious and worldly benefits, are far more than the positive [effects of lust]. The most truthful witness to that is what is known about the conditions of the various nations, and hearing the news of people. [These two things] suffice someone from having to try and witness it themselves. However, whoever has been through that would take heed in the necessary fashion; there is no lust except that its harms outdo its benefits.”
Ibn Al-Qayyim, may Allaah have mercy on him, said about lust:
“It is of [various] types, and sometimes, it could be disbelief, such as someone taking the one they lust as a rival [to Allaah], loving them as they love Allaah, so how about if they have greater love in their hearts for the one they lust than they do for Allaah! This type of lust is not forgiven for the one who does it, because it is one of the greatest types of polytheism, and Allaah does not forgive that He should be associated in worship with anyone.
The sign of this disbelieving and polytheistic lust is that the one who lusts gives precedence to the pleasure of the one they lust over the pleasure of their Lord. Further, if the right of the one they lust after and the right and obedience of their Lord are at odds, they give precedence to the right of the one they lust after over the right of their Lord, and prefer the pleasure of [the one they lust after] over [their Lord]. Also, they sacrifice and put in efforts for the one they lust after to the utmost of their capabilities, but only sacrifice in the least bit for their Lord. Further, they would use all of their capabilities in trying to please and in obedience to the one they lust, and in efforts to get closer to them, but they leave the meager remnants for their Lord.
Thus, [if you] ponder over most cases of lustfulness, you would find that it is in agreement [with what has preceded]. Then, place their situation on one hand and their monotheism and faith in another, then weigh them both in a way which would make Allaah and His Messenger happy, and in a way which is in accordance with justice.
Perhaps someone who is lusting would admittedly say that the one they lust after is more beloved to them than the monotheism of their Lord, and that being close and communicating with them is more beloved than the mercy of their Lord. We seek refuge in Allaah from such loss, and there is no doubt that such lust is of the greatest ways of committing polytheism.
Many of those who have lust admit that they have no place in their hearts for anyone or anything besides the one they lust after, period. Rather, the one they lust after has taken their heart over, making them a sincere slave to [the one they lust after] in every possible manner. Such a person is pleased with showing servitude to a created being like themselves over showing servitude to the Creator, Exalted be He. Servitude is to show complete love, obedience, and submission. Such a person has expended the strength of their love, submission and humiliation for the one they lust after, so [in reality], they have shown them true servitude. There is no comparison between the evil of this great matter and between committing a lewd or immoral action, for the immoral action is a big sin that the person does, and its ruling is similar to other big sins, however, the evil of this type of lust is precisely the [same] evil as polytheism!
Some Shaykhs used to say: ‘To be tested with an immoral action is more beloved to me than to be tested with being lustful, causing my heart to show servitude to it, and busying my heart from Allaah.’”
Al-Jawaab Al-Kaafi (pg. 150)