I always tell my S/O if some girl is flirting it up with me. Why? Cuz I'm not a sneak.
But it's more than that.
Word gets out eventually. The LAST thing you want is for that S/O to have to hear about some girl constantly trying to flirt with you from a third party...a friend or co-worker or someone like that.
Instead of looking surprised by the info and feeling embarassed about it, she gets to look at whoever is delivering the news and say "I heard about that...he told me and we laughed at it together"
That's important.
I agree.
I tell him, not because I want to make him jealous, I just think it's respectful to communicate with him so he isn't caught off guard. He thinks it a compliment when a guy hits on me but it gets disrespectful when I decline the guy and he persists.