This is only my personal opinion -- a theory if you like-- but at least in some cases of children born less than nine months after the marriage, I think the man wanted to be sure the woman was fertile before committing to her for life. Farmers in those days didn't have the tractors, etc. that farmers have now. They needed lots of kids in order to keep the farm going. (Again, this is only a theory of mine.)
I wonder if the female farm workers had any protection from the unwanted advances of their employers or the farmhands. I don't know if they had a lock on the door while they were sleeping. (Did they even have their own room or a room shared with other girls?)
Of course some girls just fell in love, just as today, and some boys, just like today, didn't want to marry after having their way with the girl.
At any rate, oäkta children were very, very common in Sweden.