The sentence means marriage is the most pleasant protection against poverty for these young women. If they don't get married, they probably won't make a lot of money, so getting married is their best option for financial security. The confusing part of the sentence is the use of the word "want" in "...preservation from want." Here, "want" means "poverty." If you are poor, you are "in want" because you want more things. "Preservation from" simply means "protection from."