8. It is better to marry an already married man rather than becoming public property
 
In Western society, it is common for a man to have mistresses and/or multiple extra-marital affairs, in which case, the woman leads a disgraceful, unprotected life. The same society, however, cannot accept a man having more than one wife, in which women retain their honorable, dignified position in society and lead a protected life.

Thus the only two options before a woman who cannot find a husband is to marry a married man or to become public property. Islam prefers giving women the honorable position by permitting the first option and disallowing the second.

There are several other reasons, why Islam has permitted limited polygamy, but it is mainly to protect the modesty of women.