Many couples stop reproducing when they have achieved a desired balance: at least one of each sex, for instance, or at least one boy to carry on the surname. At present a family with, say, four boys, will go on 'trying for a girl', and unwanted large families accumulate through long runs of the less-desired sex, only eventually brought to an end by a single baby of the preferred sex. Planned sex-ratios would therefore have the effect of reducing average family size.
Second, those groups with a strong religious preference for boys would tend to dwindle towards extinction, albeit with a dangerous intermediate phase of producing surplus cannon-fodder. The same would not be true of groups with a preference for girls, since women are the limiting sex.
Incidentally, the prissy use of 'gender' when you mean 'sex' is presumably an attempt to pander to ill-understood feminist sensitivities. 'Gender' is a linguistic technical term. Its connection to sex in the languages with which we are most familiar is undeniable. But, as Steven Pinker shows in his brilliant book The Language Instinct (Allen Lane, 1994):
In other languages, gender can correspond to human versus non-human, animate versus inanimate, long versus round versus flat, and other distinctions.
Using 'gender' as a synonym for 'sex', therefore, turns out to be offensively racist and imperialist. Out of the frying pan . . .