What is considered unacceptable flirting in a relationship?

Isnt a partner supposed to just trust you when you talk to other guy and know that friendly conversation does not mean “i want to be with him or have lusty thoughts of him”, or is it that you are supposed to be completely satisified with your partner that you never feel the need to enjoy spending time with other men or enjoy talking to other men ? Is there a clear definition of what flirting is?

Is it a sign that she is not satisified if she enjoys attention of other people once in a while?or the compliments of other people other than her partner?

Is he just insecure or is she just too attention seeking and unfit for a relationship with this particular man? can they work it out or are they destined to break up becasue they dont see eye to eye on this ?