How can one transform the intention from fulfilling desire to an intention purely for Allah when marrying, and what are the practical ways to achieve this and ensure its sincerity, especially since merely verbalizing the intention might not be sufficient?

The intention to fulfill one's desires through lawful means is a good intention for which a servant is rewarded. This is supported by the hadith: "And in the sexual intercourse of one of you there is charity..." The perfection of reward and the completeness of intention are achieved through sincerity in one's actions for the sake of Allah, along with striving against the self, seeking Allah's help, and contemplating the reward for good deeds. Thus, the intrusive thoughts that may cross the mind of a young man or anyone else do not invalidate a righteous intention if it exists.