The world has been facing a pandemic crisis caused by the SARS-CoV-2 virus, responsible for the COVID-19 disease. In Portugal, there were several waves of the pandemic, characterized by peaks of infections and deaths, as well as several measures that had an impact on the way people feel and relate. The main objective of the present investigation was to analyze stress during the pandemic, and its relationship with family functioning, perceived relational quality and online infidelity behaviors. 234 subjects participated, aged between 18 and 61 years. The study results indicate moderate to severe levels of stress, and these were significantly related to online infidelity behaviors. A total of 9.4% of the sample engaged in online infidelity behaviors during the pandemic period (between March 2020 and May 2021) and 13.4% of these behaviors were predicted by the variables under study. It was concluded that stress and relational quality are risk factors for online infidelity behaviors. Stress is also related to the dysfunctional dimensions of family functioning. In the aftermath of the pandemic, various aspects of the relationship may have been affected by the levels of stress felt, and culminated in online infidelity behaviors and family dysfunction. The results suggest the importance of relationships as a protective factor against pandemic changes, but also the need to work on relational aspects in times of crisis.