http://quran.al-islam.com/Tafseer/DispTafsser.asp?l=arb&taf=KORTOBY&nType=1&nSora=18&nAya=50

This is a report from Mujahid which is found inside the Tafsir of Al-Qurtubi, these are not the words of the prophet Muammed & this is not even the opinion of Qurtubi. This is one isolated report. A commentary of this report by Shayk Al Shanqeeti says the following:

“Will you then take him (Iblees) and his offspring as protectors and helpers rather than Me…?”[al-Kahf 18:50] 

....narrated from Qutaadah. Mujaahid said: the way in which his offspring came into being is that he inserted his private part into his private part, then he laid five eggs, and this was the origin of his offspring. Some of the scholars said that Allah created a penis on his right thigh and a vagina on his left thigh, so he impregnates himself in this manner and ten eggs are laid every day, from each of which emerge seventy male and female devils.

It is obvious that these views are unreliable, because they are not supported by any text in the Qur’aan or Sunnah. The aayah indicates that he has offspring, but as to how these offspring are born, there is no reliable text concerning that, and such matters cannot be known through mere opinions…..

….Those who interpreted this report literally (may Allaah forgive them) said that this hadeeth indicates that he (the Shaytaan) lays eggs and that they hatch. But there is nothing to indicate that these eggs are from a female, namely his wife, or otherwise. This hadeeth may be interpreted differently, because the Arabs often speak of laying eggs and hatching them by way of metaphor. So the words “he lays his eggs and they hatch” may be interpreted as meaning that he does whatever he wants to misguide and tempt people, and whisper insinuating thoughts to them, and so on, as a metaphor. The wording of metaphors does not change, regardless of the context 

www.islam-qa.com/index.php?ref=22233&ln=eng

Conclusion

Although Qurtubi's Tafsir contains a report that states that satan has offspring by having sex with himself, This is one isolated report. Commentators may just provide information, it doesnt mean its all true,its just for reading. This by no means makes it True. As Shayk Al Shanqeeti explained this opinion is unreliable, & is not supported in the Quran & Hadiths & neither by the mainstream of scholors. Yusef Ali In his commentary on the Ayah in the Quran states;

2394. Satan's progeny, we need not take the epithet only in a literal sense. All his followers are also his progeny. (R)

Shayk Al Shanqeeti states, Arabs use this language metaphorically. Christain prince responds by claiming Qurtubi is the number 1 imaam in Islam therefore he must be true. But this is not stated anywhere, its his opinion. Christian prince just picks & choses what he likes & distorts things. Also he finds a scholors opinion, & he automatically assumes this is what Islam teaches and  he uses it against muslims. This is a logical fallacy & it does not work in any debate. Muslims accept The Quran and Authentic sunnah, thats it