“Out of wedlock” describes a thing you do when you’re not married.
Well, not YOU, necessarily, and not just any old thing. You never hear about people reading books, driving cars, or eating pretzels out of wedlock. Also, if you’re a man, no one will ever say you did anything out of wedlock — even if you never marry!
“Out of wedlock,” refers to an incident involving a pregnant woman or a baby. She got pregnant out of wedlock. She gave birth out of wedlock. Children are born out of wedlock. You might have noticed that this expression contains very little information: A woman or a child’s parents weren’t married at some point.
So why bother saying it?
I can’t tell you why some old-fashioned words and expressions survive while others do not. But I can tell you where to look for clues. The words people use reflect what’s important to them. If you’re wondering why anyone would care whether women are married or not, all you have to do is pay attention to who talks about it. Maybe the other things they say will tell you what you want to know.