Infidelity is shown to be very complex between Garp and Helen. It shows itself to strike at the very heart of the marriage. Garp and Helen still love one another and they never really fall "out of love" with each another. The infidelities they experience are more akin to mistakes in judgment than anything else. For example, the misguided plan to engage in an affair with the Fletchers is done to try to save the Fletcher marriage. It might also be done to save their own. There is little in way of malevolence towards one another in their displays.
Since the focus of the narrative is Garp, his infidelities are shown to exact more in way of reflection and regret. His interlude with the baby sitter is juxtaposed to him catching the child molester. It is perceived that his writer's block is due to his agony regarding the infidelities. His struggle within the realm of infidelities can be contrasted with Helen. Her affair with Michael Milton is because she tires of Garp's irritability and his antagonisms with the world. Garp is shown to struggle with infidelity, while Helen seems to embrace it as a form of escapism from the world in which she lives.
In such a disparity, Garp is perceived in a softer light than the glare that Helen absorbs. This can be seen in Garp's demand for her to end the affair. It is a demand that comes across as Garp being wounded or hurt. While Helen would have undoubtedly suffer the same level of hurt from Garp's affairs, it is never shown to the extent that it is when Garp is wounded by Helen's affair with Michael Milton.
It is very difficult to ascertain if gender is the defining characteristic in casting the infidelities in this light. It might be more because Garp's spirit is restless throughout the narrative, struggling to find his place in the world. This makes his infidelities more of a reflection of someone not at peace with himself or the element of his world. I am not sure that there is a deliberate attempt to construct infidelity as a reflection of gender. Infidelity for Garp is different than it is for Helen. Whether this is because of gender or the difference between the two characters is largely a matter of interpretation.
This analysis can be pushed further into an even more challenging domain. Jenny's primary focus in the novel is the reconfiguration of gender roles. Jenny outwardly and defiantly challenges the idea that women have severe realities forced upon them: "either somebody's wife or somebody's whore." Garp might be an active response towards configuring anyone into such defined elements. It might be why he takes such a strong stance towards the Ellen Jamesians. Garp simply wants to exist in the world, devoid of the external conditions into which individuals re forced to conform. It might be in this light that Garp seeks to bring this transformative capacity into the world of infidelity. Perhaps, he seeks to suggest that infidelity is not something that needs to be defined by gender reality. The conditions that cause infidelities reflect individuals who are in pain, suffering in their relationships. While Garp might wish to transcend gender conditions, it is interesting to note that he capitulates to them when he demands Helen to end the affair. In this, one sees that gender is a microcosm for the transformational capacity of human beings only to be undercut by the limiting realities and paradigms that bind us all.