Musar על בבא מציעא 173:12

Our sages in Baba Metzia 86, explain that the reason the bread was never brought to the table was that Sarah had unexpectedly menstruated, become ritually unclean through having become biologically rejuvenated as of that day. 18,11 which reports that Sarah had experienced her menopause some time ago, refers to her situation prior to the arrival of the angels. It was Abraham's custom not to eat anything that was not ritually pure. Still, even assuming that Abraham personally conducted himself on that basis, how could he apply such stringent yardsticks of conduct to others? One simple answer is that it is not good manners to offer guests dishes that the host himself is not prepared to partake of. Rabbeinu Bachyah answers the problem quite differently. Sarah had stopped handling the dough the moment she became aware of her condition, since she knew that otherwise her husband would not touch the bread. Abraham therefore had no need to worry about that. However, Abraham was concerned about the חמץ aspect, seeing the date of the angels' arrival coincided with the calendar date on which Passover would occur in the future. During the time in which Sarah had to make arrangements for someone else in her household to handle the dough, the dough could have become leavened and as such forbidden. The date is alluded to by Abraham saying to Sarah לושי ועשי עגות, "knead it and make into cakes." The expression עגות, occurs in connection with מצה, i.e. עגות מצות, Exodus 12,39. Since Rashi does not follow the above line, we prefer to accept the approach of the Midrash, who regards the whole procedure as sacrificial gifts. It is of course, totally inadmissible to offer as a sacrifice something that is ritually unclean, hence the bread could not be served.