Is swearing on the Book of Allah to stop watching pornography, at the request of one's fiancée, considered associating the fiancée with Allah?

Abandoning prohibitions due to the fiancée's dissuasion is not polytheism; rather, it is a blessing from enjoining good and forbidding evil, and cooperation in righteousness. It is presumed that you abandoned the sin out of fear of Allah, and your fiancée assisted you in that. So, praise Allah for repentance, and seek His help for steadfastness. We should note that the suitor is a stranger to the betrothed until the legal marriage contract is concluded.