Apparently, cheating on your partner has a lot of basis in a person's biology, if studies are to be believed. The New York Times recently reported a study by the University of Queensland, Australia that determined whether people were more likely to be adulterous based on their vasopressin receptors. According to Quartz , Vasopressin is a hormone that affects trust, empathy, and sexual bonding in animals.
The study, conducted on 7400 Finnish people, claims that 40% of the “variation in promiscuous behavior in women could be attributed to genes.” One thing is clear - Several issues relating to infidelity as well as harmony between couples is connected very closely to a few hormones, namely, vasopressin and oxytocin.
Goes without saying though, you can't use this as an excuse for your girl/boy to take you back!