Totally agree with you on that! It's bad enough when a guy/girl flirts with someone other than their partner when in a relationship but...when you're married it's totally out of line; and in my humble opinion, not harmless at all!
Matthew 5:28
But I tell you that anyone who
looks at a woman lustfully has already committed
adultery with her
in his heart.
If all it takes is a 'look' (or a thought) to commit adultery...isn't flirting taking that 'thought' one step further into action?
But to answer the original question...
When a married guy flirts with me...I just tell them straight out "I'm sorry but what you just said is highly inappropriate for someone who is married, and it makes me feel uncomfortable...please don't say it again"
The only reason married people flirt is because
they get away with it! The more we collectively resist it...the less it will happen. Maybe?