In The Great Gatsby, chapter 2, what does it mean when he says, "The fact that he had one was insisted upon wherever he was known"?

Tom insists that wherever he goes, people acknowledge his mistress. By using the word "insisting," Fitzgerald suggests that Tom's blatant infidelity is being forced upon his friends and acquaintances. These would most likely be elites for whom his casual disregard for social norms (of discretion, not fidelity) would seem repugnant.
Tom is hypocritical, lamenting the downfall of civilization at the hands of racial minorities while ignoring the standards of social decency himself. Insisting on his own class's superiority, he brings a lower-class mistress to fashionable places and insists that his well-off acquaintances speak to a woman whom they would normally ignore.
At the same time, this puts them in a socially awkward position with regard to Daisy, whom they most likely also know and socialize with on other occasions when Tom brings her. Even Nick is curious but uncomfortable about meeting her, both drawn to Tom's recklessness and repulsed by it.
So Tom's marital infidelity is a secret to no one, including Daisy, as we see what she speaks of wanting her daughter to grow up to be stupid. It may well be behind her willingness to reunite with Gatsby. It is certainly a part of Nick's judgment at the end that these are careless people, his longing to go back West, and his desire to have the whole world stand at attention.