Is what the questioner did—actions described as "lust" and involving "a man being ridden by a woman"—considered a form of shirk (associating partners with God)?

A boy riding another boy, or a woman seen riding a man, is not polytheism (shirk), even if desire is present. The one who suffers from obsessive thoughts should turn away from thinking about this matter and occupy himself with good deeds, obedience, study, and permissible entertainment. He must avoid looking at forbidden things, and if he is able to marry, he should hasten to do so. Otherwise, he should seek help through fasting, for the Prophet, peace and blessings be upon him, said: "Whoever among you can afford to marry, let him marry; for it helps him lower his gaze and guard his modesty. And whoever is not able to marry, let him fast, for it will be a shield for him."