Is the fluid that exits when thinking about intercourse — and is thick, copious, and viscous like semen, but without pleasure — considered semen or madhiy (pre-seminal fluid), given that their appearances are identical to me?

If the fluid discharged is not accompanied by lust and is not followed by fatigue, then it is madhy (pre-seminal fluid) and not maniy (seminal fluid). It does not necessitate ghusl (definition) (ritual bath), but it is obligatory to wash the area from which it exited and whatever clothes it stained, and to perform ablution (wudu') when intending to pray.