Been in a LTR for 15months, my LTR has just asked me, "is it ok for me to go for lunch with my Serbian guy friend?"

The current state of my LTR is sex always on tap, have to turn it down sometimes, im not in the mood, but lots of arguing recently so not too great. She is a jealous type.

Is it appropriate to let my LTR go to lunch with a guy? I see no point or reason for a girl to go on lunch with a guy. Is it a shit test? What is the appropriate response? I see it as unacceptable.

She says " I met him in a club when he was visiting from Switzerland, and he was wondering if it was ok to go to lunch as he is visiting England again"

I personally see the request in the first place as disrespectful and a red flag. my answer to this question would be a resounding NO. but I would like some deeper insight as to how you interpret this situation