"Young pregnant girls?" How old is that? Under 21? If the pregnant girl is going to be a single mother and give her
surname to the baby, I can see why a boy might be preferable. The young mum may feel her parents see passing on the family name as a compensation, for their grandchild being illegitimate.
Maybe the girl is from a culture where girls marry young, and where she sees boys as being more valued than girls. So having a firstborn whose a son, gives the mother more status.
Are you implying that older mums prefer to have a girl first, or don't have preferences? Speaking for myself, I became a mother at age 29, and was realistic enough to realize there's no point in wishing for a particular gender as you have to accept whatever you get.