her mother in law Naomi was pushy and put her up to it. also Ruth had already been married so she knew what being married was like.
I would say that it was more Naomi wasnt happy without son or a grandson than Ruth actually needing a husband. Just my view though from reading the Book of Ruth.
Your ignorance of the cultural and social settings of Biblical events never ceases to astound me. Seems like every time you comment on a Bible story you make it sound like the characters had the thinking, desires and perspectives of our 21st century societies, and they just plain didn't. There're books, websites, Bible studies, plenty of resources out there to help us understand the world and society the story takes place in.
In this case, it wasn't primarily or only about Naomi getting a son or grandson (although someone to carry on the family name and inherit was so important that it was law that a dead man's brother should marry his deceased brother's wife to produce an heir for his brother (Deut 25:5-6) and the text implies something similar was expected in acquiring Naomi's husband's property in Ruth 4). The primary concern for both Ruth and Naomi was that they were widows and in that society being a childless widow meant being destitute with no economic means of support. Gleaning (why Ruth was in Boaz's field to begin with) was basically the welfare system of Ancient Israel which God had put in their law (see Lev 19:9-10). To put it accurately in modern terms, you had two impoverished women on welfare whose only option to economically better themselves was for Ruth to get married. Boaz was one of the relatives who had some sort of cultural responsibility to care for them and since he'd already shown Ruth some favor and protection, Naomi knew he was a good man and their most likely prospect for continuing to get the help they needed now that the harvest was over. And let's remember they're in Naomi's culture so Naomi is the one who knows what signals are being sent and how to send signals back.
The other comment to make is that Ruth knew that in returning with Naomi she was pretty much signing up for a life of widowhood and poverty (Jews had laws against marrying foreigners and there was bad blood between them and the moabites (Deut 23 : 3-6) which was likely a large part of why the people who heard their story had such good things to say about Ruth and respect for her. She sacrificed hugely for her faith and to take care of Naomi. Is it any wonder that God honored and rewarded her?