Why in Quran 6:101 Allah says He can't have a child without mate?

I read a christian argument that keeping aside the concept of actions befitting his glory if he wills to do something then there should be no limitations and he doesn't need to be like creation. It seems like God his making self a human and saying he needs a spouse to have a son. Why quran depicts begetting only as sexual activity which we don't or perhaps pagans would also haven't believed?