In other words, "I know you wouldn't have taken her had you known she was taken--therefore, I kept you from accidentally doing the wrong you did not intend to do (by making it impossible for you to have any sex at all)."
If he was single, it is acceptable that he would've taken her immediately, because of her great beauty, not because he was "thirsty". Isaac's messenger immediately chose Rebekah for his wife. Jacob immediately chose Rachel.
If he was not single, being a King, God apparently "accepted" that he would have had many wives and/or concubines--just as He permitted the Jews, in the Torah, the concession (and there are many in Torah) of "any cause divorce", because of the hardness of their hearts, which Jesus nullifies, rejecting it as adultery, restoring the higher standard God established in Genesis 1 (Mt 19).