Is the desire to commit a sin, while being unable to do so, considered unlawful, and is repentance obligatory for an act not yet committed?

Your resolve to commit a sin when you have the opportunity is considered a sin by many scholars and requires repentance. People have differed regarding the punishment for intention without action. One group cited the hadith: "Indeed, Allah has pardoned for my nation what their souls whisper to them, as long as they do not speak of it or act upon it," and the hadith: "If a servant intends to commit an evil deed, it will not be written against him." Another group cited the hadith: "When two Muslims meet with their swords, both the killer and the killed are in the Hellfire." It was asked, "O Messenger of Allah, this is the killer, but what about the killed?" He said, "He intended to kill his companion," and the hadith: "If I had what so-and-so has, I would do with it what so-and-so does," where those who wish for something are equal in burden and reward.

The distinction lies between "passing thought" (al-hamm) and "firm intention/will" (al-iradah). "Passing thought" is not accompanied by any apparent action, so there is no punishment for it; rather, its owner may be rewarded if he abandons it for the sake of Allah. As for a "firm intention," it must be accompanied by performing whatever is within one's capability, even if it is little. This clarifies the meaning of the other hadiths, such as the hadith of the killer and the killed in the Hellfire, because the killed person intended to kill and did what he could. Your intention mentioned in the question falls under the category of firm intention, so you must sincerely repent and resolve to adhere to obedience and avoid disobedience.