I'm not the jealous type, but I know my limits aren't other people's limits. In the case of extra-relationship flirting, there are several sets of limits in play, those of the direct participants, and those of their relationship partners if any.
Until there's evidence of the other direct participant's limits and interest, IMO it's out of line to flirt beyond smiles and eye contact and graceful language, not even flirting really but friendly polite interaction (which seems frequently mistaken for flirting). And keep the innuendo dialed down to zero.
If they seem interested, I'll add compliments and nudge the innuendo up to 1 or 2 out of 10. At that level, I feel like I'm not out of line at all, and I would think a partner who had a problem with it should self-examine to consider why. I'd place this firmly in the grey area between friendly and flirtatious.
It's hard to judge whether moving beyond that into outright flirting, perhaps with casual/innocent/not-so-innocent physical contact or increasingly explicit innuendo, is appropriate in a given situation. A partner objecting to this level of flirting is not necessarily being unreasonable, and everyone's comfort level will differ.
