Single women had 2 choices to survive, either 1) marriage; or, 2) prostitution, and Moabite women in Israel might be pushed more toward option # 2. According to the prophet Samuel who wrote this book, Rut was righteous, so how are the characters going to treat this Moabitess who chose Naami and her people over her own? Who would Rut marry? Who would she choose? Would she be accepted? Will her children be accepted?
Naami was a righteous woman and not naive. For Rut and Naami's continued survival, they were going to have to act. She in wisdom encouraged Rut toward Boaz in a rather definitive and bold move. Essentially Rut proposed marriage to Boaz by sleeping near him at the harvest campsite. Regardless of whether that means they had sex, the point is she flirted in such an explicit manner as to propose marriage to him. Boaz was thrilled by her proposal, and states explicitly he was glad to be chosen over younger men and her more immediate family redeemers. He praises her righteousness, exclaims how hard working and how loyal to her mother in law she was. Boaz could see she was modest in the fields with the young guys around. He immediately went about removing any barriers to their marriage, thus accepting her into his home and the community. This benefit of including the righteous stranger into the community was a great one that impacted not only Naami but the union was to produce kings. This fact is hidden in the name of Naami's deceased husband Elimelech (to me the kingdom).
Although she is not considered one of Israel's seven prophetesses, http://www.dafyomi.co.il/general/info/48prophets.pdf the book was written by the prophet Samuel who anointed David HaMelech, knowing he was descended from this Moabite convert. Just as the Holy One is hidden in this story, the kingship alluded to in this union. Samuel chose what seemed the unlikely choice, David haMelech, as did Naami and Rut. They looked at the inner person, not the lineage or externalities, and so can we.