Bereshit 25:19 reads:
וְאֵלֶּה תּוֹלְדֹת יִצְחָק בֶּן אַבְרָהָם אַבְרָהָם הוֹלִיד אֶת יִצְחָק:
And these are the generations of Isaac the son of Abraham; Abraham begot Isaac.
Rashi: (bold in the English is mine)
אברהם הוליד את יצחק: ... לפי שהיו ליצני הדור אומרים מאבימלך נתעברה שרה, שהרי כמה שנים שהתה עם אברהם ולא נתעברה הימנו, מה עשה הקב"ה, צר קלסתר פניו של יצחק דומה לאברהם, והעידו הכל אברהם הוליד את יצחק, וזהו שכתב כאן יצחק בן אברהם היה, שהרי עדות יש שאברהם הוליד את יצחק:
Abraham begot Isaac: ... Since Scripture wrote: “Isaac the son of Abraham,” it had to say: “Abraham begot Isaac,” because the scorners of the generation were saying that Sarah had conceived from Abimelech, for she had lived with Abraham for many years and had not conceived from him. What did the Holy One, blessed be He, do? He shaped the features of Isaac’s face to resemble Abraham’s, and everyone attested that Abraham had begotten Isaac. This is the meaning of what is written here: “Isaac, the son of Abraham,” because here is proof that “Abraham begot Isaac.” - [From Midrash Tanchuma, Toledoth 1]
Why would the scorners think that? I suppose it was well known that Avraham already had a son (Yishmael) from Hagar - so wouldn't that fact make it clear that it was Sarah that was barren?
So then I thought: 'Well, that's why they are scorners - they don't need to be logical'
But then, why would Hashem pay attention to illogical nonsense?