I think that one reason why their marriage as shown in the film works is because they demonstrate a fairly open relationship between one another. Gil and Karen are constantly talking and communicating with one another. They discuss what frustrates each other, even if it comes at the risk of alienating the other. When Karen talks about children, she does so openly with Gil. When he feels frustrated by his job, he does not hedge in talking with her about it. Both of them speak openly to one another. Even in the moments in which they are uncomfortable with the results such as the discovery of Kevin's learning challenges, they embrace communication with one another and avoid the traditional marriage pitfall of repression and silence.
When Tennessee Williams speaks of how marriages are intrinsically driven by "mendacity" in A Cat on a Hot Tin Roof, one sees that Gil and Karen are not governed by such conditions. Both of them live in an emotional climate where there is complete openness in discussion, where both of them display their weaknesses to one another. I think that it is because of this that their relationship is so strong and one of the reasons why theirs is the most consistent throughout the film.