I wasn't familiar with the expression, but apparently it means having one boy and one girl (ideally, the boy is first). The male serves as an heir, and the female can be married off to a good family.
|Gentleman's family...for now.|
My impression is it was used by Irish Catholics to subtly mock Protestant families — first of all because Protestants would use contraceptives to have smaller progenies, but also because it implies that a gentleman could simply will himself to sire a child of each gender.
If this is the ideal scenario, we really blew it. In addition to pushing us several rungs down in the child-configuration rankings, our third offspring means we're no longer a gentleman's family.
I guess I can live with that.