How do we know Pharaoh did not have relations with Sarai?
"So it was, when Abram came into Egypt, that the Egyptians saw the woman, that she was very beautiful. The princes of Pharaoh also saw her and commended her to Pharaoh. And the woman was taken to Pharaoh's house. He treated Abram well for her sake. He had sheep, oxen, male donkeys, male and female servants, female donkeys, and camels. But the LORD plagued Pharaoh and his house with great plagues because of Sarai, Abram's wife. And Pharaoh called Abram and said, "What is this you have done to me? Why did you not tell me that she was your wife? "Why did you say, 'She is my sister'? I might have taken her as my wife. Now therefore, here is your wife; take her and go your way." So Pharaoh commanded his men concerning him; and they sent him away, with his wife and all that he had" (Genesis 12:14-20).
While Pharaoh had brought Sarai to his house, notice that when he scolded Abram he stated that he "might have taken her as my wife." In other words, he had not yet done so.
I do not see that "might" in most versions and in the Strongs I don't see it indicated that it was in the original text. When Abram takes Sarai to Abimelech's kingdom later in chapter 20, the message is clear that she had not been touched, so it goes to reason the same is true here too. But I do not see the word "might" anywhere but in the King James. I am still needing help!
Actually, it does appear in the New King James Version as well, since that is the one I quoted from. You are correct that "might" doesn't appear in the text, though I don't think the translators were wrong in inserting the word for clarity.
Because the ancients treated an engagement as equivalent to a marriage, we cannot assume that just because Pharaoh took Sarai to be his wife that it implies that they had actually married at the time, let alone consummated the marriage. It was Pharaoh's intention of marrying Sarai, that is why he took her and gave Abram dowry gifts. But the return of Sarai to Abram with nothing more than a strong tongue lashing is taken to imply that there wasn't enough time for more than a plan for marriage.
Still, it doesn't change matters. Sarai was over 65 at this time and did not give birth to Isaac until she was 90.