Derbstyron's critique is well taken, but even if you developed it from there, it would only tell part of the story.
What is it about their marriages that makes these characters so attractive to each other? I'm getting too many mixed signals. Do one or both of these people just have habitual commitment problems? Alex doesn't seem to be too thrilled about her husband. Is there no passion in their relationship? If not, why is the reader told, "they both loved the people they were married to"?
Why is Mike taking being dumped so well? This relationship either doesn't mean a lot to him or the subject of breaking up has been brought up before. Why is he upset enough to cry and not ready to fight to hold on to what he's got? I'm getting mixed signals about him, too.
Reading this gives me a sense of having listened to a popular song, something like "Let's just kiss and say good-bye". The reader is given less information about the characters as human beings than about their immediate feelings in a vignette. Is there a story behind it? What motivated them to have an adulterous relationship in the first place? What is motivating them to break it off? Fiction should give the reader a better sense of what is going on inside the characters' heads than a popular song does. Here, the reader is not given that information.
One piece of advice when writing about a subject like this: develop a broad, romantic morality to guide you. By that, I don't mean something as trite and simplistic as what we might expect to find in
Leah's Way after reading Steph's review. Mine is that a human being is at once an individual with basic needs and a social creature; that without being so, our species would have long ago become extinct; that much of the suffering in the world is a result of tensions that arise from these two often conflicting human aspects; and that such conflict often takes place within a single breast. It is a romantic moral outlook that allows the writer to explore a subject like adultery without prejudging the adulterers, as a classical moralist would.