Source of Generations after Adam
5-2-2002 | IslamWeb
Question:
One of my friends had an argument with his Buddhist friend, his question hurt my friend's feelings very much but he had no answer for the question. So please shed some light on it. In Buddhist philosophy there is no god, every thing came from nature. According to their thoughts our concept of God and prophets are just fictions and fairy tales. He asked me about the evolution of mankind so I explained that from Adam. Then he posed a counter question that if Adam had only two sons according to the Holy Book of tours, after them how did the generation progress without a woman with whom they had intercourse to get children? The only woman available at that time was Eve, the mother of Abel and Cain? So his indirect question is whether those two sons had any sex with their mother? I said may be they had a sister. Then he asked me 'whether that was incest or not? Incest is a grave sin in your religion. Why did your God not create another two women for them? In this case your God is filthy. This last statement hurt my friend's feelings very much. Please send me an answer as soon as possible, with the references if any.
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
Allah Says (what means): {…To each among you, We have prescribed a law and a clear way. …} [Quran 5:48]
This verse states that Allah has given each nation a code of law and a program for action. He makes lawful what He so wishes and makes unlawful what He so wishes. He has done that to distinguish between those who obey Him and those who disobey Him.
Ibn Katheer 
said: “Commandments and prohibitions are different in codes of law. So, you find something in this Sharee’ah (legislation) that it is unlawful while you find the very thing lawful in another Sharee’ah, and vice versa some ruling here is rigid while in another soft. It is governed by the Divine Wisdom and the conclusive argument.”
And your question is related to the above quotation; i.e., it is the Divine Wisdom.
Anyway, Adam 
was blessed with twin children, male and female, at each time, and Allah had legalized to join in marriage a female of the first twin to the male of another twin to increase people on earth. Then, He prohibited such a marriage when His aim is achieved. Allah does what He Wishes and dictates what He Wishes, although He is able to create so many males and females, and to guide them to marry one another, without need to what I mentioned concerning twins.
But, Allah, in such above-named situation, has clearly stated the unity of human origin, which necessitates equality among all human beings from all human sides. This leads us to say that no one has superiority to another due to color, race, blood or nationality.
All human beings were created from one and same origin.
However, it is your duty to refer such a doubter to specialist scholars in your country to clarify to him his ignorance and misguidance, and consequently, to keep your belief and creed safe.
But, if discussion with such a doubter may lead him to blaspheme Allah, then you should not sit with him at all, unless he demonstrates good manners and politeness with Allah, The Lord of the Universe.
Allah Says (what means):
• {And insult not those whom they (disbelievers) worship besides Allah, lest they insult Allah wrongfully without knowledge. …} [Quran 6:108]
• {And when you see those who engage in [offensive] discourse concerning Our verses, then turn away from them until they enter into another conversation. And if Satan should cause you to forget, then do not remain after the reminder with the wrongdoing people.} [Quran 6:68]
Allah knows best.