It's not a sin, at least NOT to me.
As far as I'm concerned I assume there were plenty of reasons. Fist of all back then it was not as easy as it is today to determine e.g. paternity, so you might have end up confused on who's the actual father(if the child came fully after its mother).
Secondly there is economic reasons, father was the breadwinner, mother at home. Women rarely had a chance to make their own money, if they were abandonned they could either beg or prostitute themselves, not really the best options.
I guess today the idea that a woman has to be a virgin priour to marriage is more or less culturally motivated, although some religions still claim it's sinful to commit adultery.