It might be that Brahma wanted to warn his subordinates (by being sexually inclined to his own daughter) about human frailties in their dealings with women. This is always very dangerous for persons who are on the path of self-realization
SB Canto 3
Thus, finding their father so deluded in an act of immorality, the sages headed by Marīci, all sons of Brahmā, spoke as follows with great respect.
The sages like Marīci were not in the wrong in submitting their protests against the acts of their great father. They knew very well that even though their father committed a mistake, there must have been some great purpose behind the show, otherwise such a great personality could not have committed such a mistake. It might be that Brahmā wanted to warn his subordinates about human frailties in their dealings with women. This is always very dangerous for persons who are on the path of self-realization. Therefore, great personalities like Brahmā, even when in the wrong, should not be neglected, nor could the great sages headed by Marīci show any disrespect because of his extraordinary behavior.