The failing birthrate is named as one of the [contributing causes of the collapse of the western Roman empire](http://www.rhm.uni-koeln.de/128/Devine.pdf). However, in later, medieval times, all sorts of pirates, Vikings and others were involved in the supply of women, as the mortality rate of women was high then and there was a demand for them (obviously). Therefore, why did the Romans not import women from abroad or adopt legislative measures (like legalized polygamy, ...) to remedy the problem?

I'm asking, because there exists a similar situation today.