The failing birthrate is named as one of the contributing causes of the collapse of the western Roman empire. 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?
EDIT: To my fellow armchair historians. The falling birthrates in ancient times were caused mostly by deaths at childbirth. A good example of this would be Julius Caesar's daughter, Julia, who died in childbirth. It must have been so, as this problem persisted into the modern age. The baths idea is interesting but, IMO, not that relevant. It lists interesting sources, though, saying that there was a problem. Deaths, of course, created a need to replace women who died and women were being sold and bought all the way to the 19th century and, in some places, wives are sold and bought even today (Africa, Asia).