In Chapter 6, what is ironic about Tom's remark that women run around too much and meet the wrong kind of people?

Tom makes this remark at Gatsby's house one Sunday afternoon during a break in a horse riding jaunt with two of his wealthy friends, a Mr. Sloane and a "pretty woman" who is not named, but who, Nick mentions, "had been there previously," probably at one of Gatsby's parties. Nick went to pay Jay a visit at this time and was surprised to see Tom and his companions arrive a few minutes later.

After a brief introduction and a bit of conversation, Jay informed Tom that he knew Daisy. Tom abruptly dismissed the remark with a, "That so?" without further enquiry.

The lady later enthusiastically invited Jay and Nick for supper, to Mr. Sloane's great perturbation. When Nick declined, she focused on Jay and insisted that he join them. Jay accepted and promised to follow in his car. He excused himself and went inside to get light attire. Tom sneeringly remarked that Gatsby was coming and should realize that the lady did not want him. When Nick corrects him, he then, like a spoiled brat, realizing that he couldn't get his way, makes the remark mentioned in the question. He wonders aloud where Daisy had met Jay and then says:

"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.”

It is ironic that Tom should be so judgmental, since he had been involved in all sorts of sordid affairs with just such women since his marriage to Daisy, and was at that point involved in an affair with Myrtle Wilson, a married woman. He is, therefore, ironically and unknowingly, also referring to himself as a "crazy fish." Secondly, his wife is also involved in a similar relationship with Jay Gatsby, a fact that he will soon discover to his shock. His allusion therefore ironically also includes both his wife and Gatsby.

A further irony is established by the fact that Tom is, at this point, sitting on a horse and one could suggest that he should, both literally and figuratively, "Get off his high horse" and not deem himself better than others because he is being a hypocrite. The passage once again emphasises Tom's supercilious nature.