Why did Avraham not consult Sarah like he did with Pharaoh (in Parshas Lech L'cha)?
Rashi: Having suffered once at the hand of Pharaoh, he did not think she would agree to go through the same ordeal again.
Ramban: There is no comparison between the two cases, because, the moment Avraham arrived in Egypt, the people reported the beautiful Sarah to the princes and to the king, since they were a depraved society. Hence it was necessary to consult Sarah, due to the imminent danger involved. Not so Avimelech, who was a decent king 1 as were his subjects. 2 And Avraham only referred to Sarah as his sister in order to be on the safe side. 3
See also Ibn Ezra on Pasuk 3.
As is evident from the fact that, unlike the episode in Egypt, the people were not involved in Sarah being taken to the king.
The Or ha'Chayim gives a similar answer, but based on the fact that the Egyptians, who were black, were not used to seeing a beautiful woman, whereas as far as the P'lishtim (who were white) were concerned, a beautiful woman was not uncommon, and was therefore nothing to get excited about.
Is it not incredible that, at her age (she was over ninety), Sarah Imeinu should still be attractive in the eyes of kings?
Ramban #1: It was indeed a miracle.
Ramban #2 (citing Bava Metzi'a, Daf 87a): When the angel informed her that she would bear a son, her original youthfulness Was reinstated.