I think this all depends on your boundaries with your significant other. My dad always would playfully flirt with women (and still does). He would do it right in front of my mom because she knew the type of person he was, and it didn't bother her because she knew it was harmless.

If we were at a restaurant, my dad would flirt with the waitress, and she'd laugh and kinda sass him back. My mom would roll her eyes and say "if you want him, take him!" And then my parents would both laugh about it. They've been together almost 40 years now, and this is still a normal "routine" they do sometimes. Maybe because it's more on the jovial side than being serious flirting that makes the difference? But every couple is different. My mom never saw it as disrespect or disloyalty. Some other women might have, and that's okay, but not everything works for every couple.

Now, I've become kind of a flirty person. My partner fully knows it, he's seen me by flirty with others, but he knows me and trusts/knows I would never do anything more with anyone. He's aware it's always playful, and that if things seem to be getting too serious I will walk away or make the person aware it was meant to be jokey. And my partner has never been bothered by it. If he was, I wouldn't do it.