There isn't much of a relationship between Curley's wife and any of the men on the ranch with the possible exception of Slim. Most of the men see her as potential trouble because she is flirtatious and her husband is jealous and violent. Several of the men refer to her as "jailbait," indicating that she is both promiscuous and underage. Judging from what she reveals to Lennie in the barn before he kills her, she has been trying to escape from her home since she was fifteen and only married Curley because he would take her away. She reveals little information about her life between the time she was fifteen and her marriage to Curley, which suggests that she is now only around sixteen. Steinbeck would have wanted her to be quite young in order to explain her reckless behavior and to make it seem plausible that Lennie could kill her so easily by shaking her. She is small and frail, not yet fully mature. This fits the plot because Curley is small and has an inferiority complex; it can be understood why he might marry a small, young girl.
As far as her relationship with Slim is concerned, it would appear that there is an attraction between them that could lead to an adulterous affair. Slim is a big man and not the least bit afraid of Curley. Neither is he afraid of losing his job, because he is a skilled worker who could find other work easily, and he knows it. The others are either just working stiffs who can be replaced easily, or in the cases of Candy and Crooks they are badly physically handicapped and lucky to have any jobs at all. There was no government safety net in those days--no unemployment insurance, no Social Security, no Medicare, no protection from starvation. These men are about as low on the economic ladder as it is possible to get, and they are far more interested in survival than in sexual adventurism.