The original legal definition of bastard is any child born outside of marriage. I would assume the reason why it applies mostly to sons is because the law used to prevent bastards from receiving inheritance and royal titles. Daughters already had difficulty receiving these things, so a bastard daughter wasn’t much different than a legitimate daughter.
In modern times, almost all legalities around bastard children have been removed from law since most children are bastards.
I think you’re misinterpreting some of that info. There are a number of countries where children born out of wedlock are the majority, primarily in Latin America and Western Europe. In America, it seems to be hovering around 40% for the last 8 years and worldwide it’s about 15 percent, with Asia doing a lot to keep that number down.
