Tom says that "By God, I may be old-fashioned in my ideas, but women run around too much these days to suit me. They meet all kinds of crazy fish." Tom says this after it is revealed that Daisy, his wife, knows Gatsby. Tom instantly takes a disliking to Gatsby and is jealous of Gatsby's relationship, which Tom doesn't quite understand yet, with Daisy.
The irony is that Tom, while criticizing women who run around, has an adulterous affair with Myrtle. This affair is only the last in a string of affairs with other women that apparently started soon after he was married to Daisy. Shortly after they were married, according to Jordan, Tom was in a car accident with a woman who was a chambermaid in Santa Barbara, where Daisy and Tom were living. Daisy has to live with the knowledge that Tom is continually unfaithful to her, but he, in a sexist and cruel way, believes that women should be faithful and should not move much beyond the world of their husbands.
In Chapter 6, Tom says, "By God, I may be old-fashioned in my ideas but women run around too much these days to suit me". He is suspicious about how Gatsby met Daisy, and, "perturbed at Daisy's running around alone", is sure to accompany her to his next party.
Tom's comment is ironic because it exposes so blatantly the double standard he holds. In Chapter 1, he made a similar comment about Jordan Baker. At the time, Tom had a mistress who had been calling the house repeatedly during Nick and Jordan's visit with Tom and Daisy. Everyone knew about her, and just minutes before his observation about Jordan, Tom was upstairs arguing with Daisy about his mistress, and Jordan was eagerly trying to eavesdrop, salaciously hoping to get the sordid details of his latest infidelity.
Apparently Tom had always considered it acceptable for him to "run around" as much as he wants. In Chapter 2, he showed that he saw nothing wrong about making arrangements to see his mistress right under her husband's nose, and in Chapter 4, Nick revealed that immediately after his South Seas honeymoon with Daisy, Tom was involved in a scandal with "one of the chambermaids in the Santa Barbara Hotel".