Is ghusl obligatory, or is wudu sufficient, if semen mixed with pre-seminal fluid is discharged after arousal and before prayer?

Ghusl is obligatory if semen is seen and its emission is confirmed, based on the Prophet's (peace and blessings be upon him) saying: "Water necessitates water." Your prayer in this state is not valid, due to his (peace and blessings be upon him) saying: "Allah does not accept prayer without purification." What is obligatory upon you is to repeat that prayer. Your ignorance of the ruling does not preclude the obligation of repetition, although it does mean that you are not sinful.