Why does John Steinbeck portray Curley’s wife as a threat?

Steinbeck was strongly suggesting that George was going to have to do something with Lennie because Lennie's interest in petting soft little animals was evolving into a sexual interest in little girls. Lennie doesn't understand his own impulses. He attacked a girl in broad daylight on the main street of the small town of Weed. He told George he only wanted to feel the material in her red dress. But Lennie lies to George all the time, and George, who wasn't present when the incident started, has only Lennie's word for what happened. If Lennie only wanted to feel the girl's dress--which was bad enough!--then why wouldn't he let go even when George was beating him over the head with a fence picket? 

Lennie does something similar with Curley's wife in the barn. He starts feeling her hair with her permission, but then he won't let go and she starts screaming and he ends up killing her. This looks like a case of murder in connection with attempted rape and makes George recall the Weed incident.

"I should have knew," George said hopelessly. "I guess maybe way back in my head I did."

What should George have known? He should have known that Lennie was a potential serial rapist and killer of underage girls. Why didn't Steinbeck make this more explicit? Because in the 1930s it would have been impossible to get such things printed. Steinbeck wanted Lennie to kill a girl at the ranch, and he wanted her to be as young as was logically possible. Curley's wife was only fifteen or sixteen. She tells Lennie she wanted to run a way with a man when she was fifteen and she was hanging around a dance hall in Salinas. She married Curley right after that.

Steinbeck portrays Curley's wife as a threat because he wanted a very young girl for Lennie's victim. He wanted Lennie to kill her so that George would end up killing him. He wanted George to kill Lennie to create a "shotgun ending" for the novel. He wanted the novel to end quickly because he intended to convert it into a play which would appear in New York in 1937, the same year the book was released. The play could not be longer than about an hour and a half, with perhaps one intermission. So the book, which Steinbeck called "a playable novel," had to be short. It reads like a treatment for the play. The dialogue is all contained in the novel, and the exposition is all contained in the dialogue; so it would be easy to convert it to a script for a stage play. The book is about migrant workers laboring in the fields, but there are no scenes of men working in fields or doing anything outdoors. Even when they pitch horseshoes the narrative only describes the sounds of horseshoes hitting the metal stakes. Nearly everything takes place in a bunkhouse or in the barn, where Crooks' little room is adjoining. It is intended for a low-budget production in New York. The scenes by the riverside campsite could be represented on a bare stage.

Lennie's wife is a threat because she is "jailbait," that is, she is underage and could get a man sent to prison for statutory rape. And she is a threat because she acts sexy and flirtatious. The men misunderstand her. She wants to be a movie star and is only trying out her charms on these workmen, the only audience available. The fact that she is so young explains why she doesn't know better than to get too friendly with Lennie and invite him to feel her hair. If she were more mature she might have handled the situation diplomatically. Instead of screaming, she might have spoken gently and distracted him. There seems little doubt that he would have ended up trying to rape her if she hadn't started struggling and screaming. 

But she was doomed from the start, because Stainbeck, the creator of all these characters, wanted Lennie to kill her, so that George would kill Lennie, so that the book, which is obviously very skimpy, could be converted immediately into a script for a stage play which would only run for perhaps an hour and a half. 

Steinbeck never specifies the age of the "girl" in Weed, but there is reason to believe that she was very young. (Someone has commented that a red dress might have symbolized that she was a loose woman. It could also suggest that she was just a little girl.) When George is berating Lennie about the incident that almost got them both killed by an angry mob: 

He took on the elaborate mananer of little girls when they are mimicking one another. "Jus' wanted to feel that girl's dress--jus' wanted to pet it like it was a mouse-- Well, how the hell did she know you jus' wanted to feel her dress? She jerks back and you hold on like it was a mouse...."

Twice George equates the dress with a mouse, seeming to show that Lennie is graduating from little animals to little girls. Why would George take on "the elaborate manner of little girls when they are mimicking one another" unless there was a little girl involved? Lennie was not attracted to the dress. He was lying about that. He was attracted to the girl, and he might have intended to tear the dress right off her. He has a child's mind but a grown man's sexual impulses. Steinbeck made Curley's wife as young as he logically could if she were going to be there because she was married.  Lennie is attracted to young girls because of his child's mind. He probably wouldn't be attracted to grown women. 

Some readers may reject this interpretation because they like Lennie and feel sorry for him. But they shouldn't feel too sorry for him. They ought to feel sorry for the little girls he might have attacked and murdered in the future if George had helped him escape from Curley's lynch mob.