There are at least three reasons why Candy says to Curley's wife that he does not want any trouble.
First, Candy has been around the ranch for a long time. So, he knows Curley's character. Curley is a hothead who leads with his emotions and pride. Therefore, if Curley suspected his wife fooling around with the other men, there would be trouble.
Second, we need to realize that Curley (even though he is not respected) is the son of the owner of the ranch. So, to get on Curley's bad side is not something that any of the men want.
Finally, this conversation between Candy and Curley's wife happens after Lennie broke Curley's hand. So, there is a certain amount of "heat" already. So, Candy does not want Curley's wife to pry into matters.
Here is the quote:
Candy laid the stump of his wrist on his knee and rubbed it gently with his hand. He said accusingly, “You gotta husban’. You got no call foolin’ aroun’ with other guys, causin’ trouble.”