If you have a S/O, is any flirting acceptable and if it is, how much or how far is acceptable?

More on the flirting issue. I’d always thought a little minor flirtation was ok, as long as all parties know it’s staying at a minor level. I could see where this could be dangerous, so I was always careful to keep it under control. But is it a zero tolerance issue? I meant no disrespect to my s/o, but is that not the case? Your thoughts.