Answer
(Fatwa: 507/507=M-1429)
If, due to hearing about sex, semen is discharged with lust (the sign of it is that it is thick and one feels relaxed after ejaculation and excitement turns into coldness), then ghusl will be obligatory. If the wetness is not thick but it is thin and excitement does not become dull after discharge rather it increases, then that is mazi (pre-seminal fluid), because of this only wudu is broken, ghusl will not be obligatory.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband