The music is, "I Feel Pretty," I wonder if there will be any male babies in the commercial. No. There are not. Boys cannot feel pretty.
Voice over talks about how the diaper keeps your baby dry no matter how "he" moves.
Let's recap. There's not a lot to recap because there's not a lot there.
1 Pretty babies is a group that consists entirely of female babies. Baby boys cannot feel/be pretty.
2 Your baby is a he.
3 Even though the images are all of shes.
So, male is default even when talking about a group composed exclusively of females.