Commentary for Genesis 26:8

ויהי כי ארכו לו שם, after a while Yitzchok was no longer careful to conduct himself in a manner that the suspicion that Rivkah was his wife could not be aroused. The reason was that he did not think that she would be abducted.

מצחק את רבקה אשתו. making sport with his wife Rebeccah. He indulged in the kind of affectionate behaviour customary between man and his wife. If we are to accept the interpretation of Bereshit Rabbah 64,5 that Avimelech observed Isaac making love to his wife, we must assume that this took place under circumstances when marital intercourse is permitted even during the day, such as for therapeutic purposes (compare Maimonides Hilchot Deyot 40, or the example quoted in Ketuvot 65 about the widow of Abbaye who inadvertently caused Rava to become aroused so that he had to go home in the middle of the day in order to have marital relations with his wife). The righteous endeavour not even to commit the semblance of an unbecoming act.