It's wrong to lie to your significant other.
Joe is dating a jealous person, and he has been hanging out with a female friend all day.
Joe wants to avoid fighting with his partner.
Joe tells his partner he was hanging out with a friend and omits information about her gender.
Joe makes up a story about what he has been doing all day.
Joe's partner is satisfied with the answer and doesn't pry any further.
Joe's partner finds out the truth and becomes even less trusting of him.