What do you do if sperm comes out after ghusl?

Does urinating before ghusl matter? Do I have to repeat ghusl if left over sperm comes out after I performed ghusl?

Salam Alaikum,

It is not sufficient unless the ritually impure person urinates before taking the purification bath.

the ejection of semen due to lust is what requires Ghusl, if it comes after Ghusl without a sense of lust then you simply wash and make wudhoo for it.

What does one do if they were unaware of this ruling (urine before ghusl) and prayed for their whole life without doing it?

Imam Qasim, peace be upon him was asked a similar question.

He was asked about a person who was a novice and unaware and not taught the ritual ablution, how to free himself of urine, or the impurity of wine and intoxicants. Is it obligatory for such person to renew his prayers while he was in that state?

He replied : One seeks repentance to Allah from past sins And shortcomings and upholds that which Allah has commanded him, by upholding that which was commanded to him from the prayer rites And otherwise, this would serve as recompense, Insha Allah. There would be between him and Allah a sufficient repentance alright.

In short : The person seeks repentance and starts following the correct methods and he does not have to repeat all those prayers. 

Ref :  Masaail Imam Al-Qasim