By far, the most surprising opinion can be found on the Bar Ilan website, where Dr. Joseph Fleischman argues rather convincingly that Yitzchak was sexually abused. Money quote:
In my opinion, the plain peshat meaning of metzahek agrees with Rabbi Akiva, meaning to do something forbidden in the realm of sexual behavior. In biblical Hebrew, the verb tz-h-k means both to laugh, joke, play and amuse oneself, as well as to enjoy oneself sexually. Similarly, the Akkadian verb sahu means to laugh, smile, be alluring, entice a person to sexual actions that go against the accepted norm (cf. Chicago Assyrian Dictionary, S, pp. 64-65). Thus we may say that the Akkadian verb sahu and the biblical Hebrew tz-h-k are etymologically parallel.I hestate to psychoanylize the Avot, but it's been remarked by others that Yitzchak is, by far, the most reserved of our patriarchs. He's quiet, sickly, and aside for one episode with Avimelech, very much a background actor. Some have said the trauma of the Akeida is what produced this personality. To this, Dr. Fleischman's shocking suggestion adds an interesting wrinkle.
In the story at hand it seems metzahek should be read as a euphemism for some sexual act for the following reason: metzahek must refer to some extremely grave act performed by Ishmael, sufficient to explain and even justify Sarah's harsh demand.
The Greek Septuagint and the Latin Vulgate add the words "Sarah saw Ishmael playing with her son Isaac." Quite likely the addition of these words to verse 21:9 stems from the fact that the Masoretic text without the indirect object appeared incomplete to the translators. If, however, we take metzahek as referring to some sort of sexual act, the verse is complete as it stands; Scripture notes only the fact of his illicit activity but does not go into detail.