Is flirting ever harmless? I don't think so. There is a reason why flirting is done..to attract a mate, to give a boost to the self esteem, to banish boredom from a conversation. But IMO it is never done without a reason and requires forethought, be it subconscious or conscious. To be secure in a relationship, secure enough where there is no need to attract a mate, no need for a self esteem boost, and no need to use flirting as a means to jolt the conversation, would be the optimum situation. Even though everyone in your scenario knows where they are going at the end of the day, why are they making a decision to engage in behavior that suggests insecurity? You already have a mate...do you feel the need to line up more potentials..in case? Does flattery from unknown men or sexual innuendos from a friend enhance your personal self esteem? Do you lack sufficient conversations skills to turn the conversation to a different subject that does not involve flirting? I sit here and answer this post thinking about myself, my own behavior. Flirting puts me sort of out there, in a risky situation. The risks are that i will be ignored, or not. If i am ignored, well, my self esteem suffers. If i am not ignored, then my behavior has a meaning. But i am not in a secure relationship with my partner by my side. If my partner flirted, I would ask him/her why they felt it was necessary to bring that kind of attention to themselves, what meaning is behind their actions.