Is what a person does in terms of sexual fantasies that lead to pleasure, and the accompanying movements on the bed while imagining the other party, considered an act with a religious ruling, and does it have an effect on the reproductive organs, the mind, or the body?

These thoughts are satanic whispers (waswas), and you must seek refuge in Allah from them, detest them, and cut them off from yourself, for they are the root of corruption. Warding them off in their beginning is easier than warding them off after they become a will and a determination.

The actions resulting from indulging in them are sins for which the doer is held accountable.

You must remember that Allah is aware of you, and you should revere Him and feel shy before Him. Occupy your soul with thoughts of faith, righteous deeds, and beneficial knowledge. Stay away from provocations, maintain modesty by lowering your gaze, and avoid bad company.

Marriage is recommended for those who can afford it, and fasting for those who cannot, as the Prophet, peace and blessings be upon him, said: "O young people, whoever among you can afford to marry, let him do so, for it helps in lowering the gaze and guarding one's chastity. And whoever cannot afford it, let him fast, for it will be a shield for him."