1. Fancying people more after you sleep with them.
2. Craving carbs.
3. Letting your eye wander while you're in a relationship.
Feeling sexually attracted to other people once you've been in a relationship for a while is biological, not immoral. According to author and professor of anthropology Meredith Small in her book Female Choices: Sexual behaviour of female primates (Cornell University Press, 1993), “The search for the unfamiliar is documented as a female preference more often than any other characteristic our human eyes can perceive.”
See? In their book Sex at Dawn, researchers Christopher Ryan and Cacilda Jethá argue that we might actually be designed to have sex with a number of partners because it means we get the strongest, healthiest baby possible and the males in the community are less likely to harm any children in case they’re their offspring.