Ha! The other night I was at the bar in a mixed circle with my lady. I was in conversation when I feel her grab my arm and pull me close.
Later she tells me a younger girl was walking by and gave her a flirty smile and said to her "well Helllloooo..." When my lady took my arm the would-be lesbian suitress looked disappointed and said "oh..." and walked off down the bar.
Then there was the time in the Phoenix airport I was walking down the Terminal as a flight had just disembarked. The flight crew was leaving the gate when a stewardess made laser eye contact with me and then looked me up and down, head to toe. My girl saw it, gave her a smile as the stewardess gave her a little nod about 1 1/2 steps further she tells me.. "That girl just totally checked you out!" while I acted completely unawares. (Yeah I saw it....).
Point is sometimes showing your partner they are with someone desired by the opposite sex is a little bit of validation for the person you are with. Social proof within the relationship.
Now I've also been at an event where a girl got mad her boyfriend wasn't paying attention to her so she came and danced with me and that almost got me in a fight when he found out. Meanwhile I was unawares...