I dont see what the big deal is, as long as the couple is okay with it, why should what an outsider says matter?
I personally dont find flirting itself as harmful to a relationship as much as if one partner were deliberately doing something that would be hurtful to the spouse (which can be pretty much anything). That shows more of a lack of respect and care for the person, than just flirting alone does.
But that’s just me. :k: