There has to be some (dis)incentive to have children without marriage in some contries and not others.
I Germany I guess people think you might as well marry when having children because you get extra money, less taxes whatever and maybe that's not the case in other countries.
Marriage is a civil institution in Germany. A church marrying you has exactly as much legal power as a random citizen doing it: None. You get married at the civil registry office, by a bureaucrat (but yes they're amenable to some mild ceremony)
Another very important factor is that in Germany it's extremely difficult to become the official father of a child when you're not married to the mother. This obviously comes with a lot of problems. For example when the mother suffers complications during birth. It's just way easier to marry instead of doing all of that paperwork.
Yeah, in Germany you get tax benefits under certain circumstances when married. In many other countries (e.g. Austria), marriage makes no difference. That's already a strong motivator.