Having just informed us that they males and females left the Ark separately, what does the Torah then mean when it writes that they all left according to their families?
Rashi: It means that they undertook to stick to their own species from then on.
Rosh: The animals reproduced, but the people did not (as yet). Refer to 8:18:1:3.
Da'as Zekenim: The animals that had stuck to their own species merited to enter and leave the Ark.
Rashi writes: "[The animals] accepted upon themselves the stipulation that they mate only within their own species." However, according to one explanation in Rashi above (6:7), it was not the animals that corrupted their ways, but people who had cross-bred them?
Mizrachi: According to that approach, we must explain that the animals accepted not to allow people to cross-breed them. 1
Gur Aryeh: Even according to that approach, once people initiated the cross-breeding, the animals (prior to the Flood) continued to do so of their own accord. Now, they accepted to stop doing so. 2
רש"י: קבלו עליהם על מנת לידבק במינן: קשה שללשון אחת ברש"י בסוף בראשית (ו,ז) לא השחיתו החיות את דרכם אלא בני אדם הרביעו מין על שאינו מינו?