In The Great Gatsby, Catherine says both Tom and Myrtle want to divorce their spouses. According to Catherine, neither Tom nor Myrtle "can stand the person they're married to" and "If I was them I'd get a divorce and get married to each other right away."
Catherine then goes on to say that the reason Tom cannot get divorced is because Tom's wife Daisy is a Catholic and Catholics "don't believe in divorce."
This is not a true comment and perhaps a lie Tom tells Myrtle so she won't pressure him to make a longer-term commitment. Later on in this scene, Tom smashes Myrtle's nose because she keeps saying Daisy's name. Both of these facts suggest the true nature of Tom's relationship with Myrtle: he wants to sleep with her and take her around town, but would never forge a longer-term committed relationship with her and does not really care about the consequences his relationship with her might have.
The relationship between Tom and Myrtle is symbolic of one of the novel's larger themes: the way the rich "smashed things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they had made..."