1- Yes, determining the father is an important factor. If a woman has multiple partners, the father may not be known. Yes some people have access to DNA testing today, but in the past people did not, and even today the majority of people do not have access to such testing or cannot afford it.
2- Men and women struggle equally, but in different ways. Both have physical needs, but men’s physical needs are different. There are biological differences between males and females. These differences are proven by science.
3- One reason for the temporary marriage is to take care of a woman who cannot take care of herself (like a widow). Historically, this mainly applies to men because they are in a better financial position to take care of other families/women. It is much less common for a woman to financially take care of multiple men besides her husband. So there was no social need for her to marry more than one man. Now why would a man conduct temporary marriage with a woman he wants to take care of and not a permanent marriage? One reason could be to avoid her inheriting from him because this could lead to tension with his first family. If he dies and this other lady comes asking for inheritance, this could lead to a major dispute. So some men want to avoid that situation. There are other reasons such as spending money and time equally among the wives (in a temporary marriage this would not be a condition that he would have to observe).