Flirting

I don't understand why people see flirting as so bad in a relationship. Yes, you shouldn't talk sexual to anyone else. But whats the real definition of flirting?? How do you know when you're crossing a line?? When you invite them to have sex. I know people who have been married for decades and they'll flirt right in front of their wife jokingly. I wouldn't want my man flirting with other woman but like i just feel like people make too much of a big deal out of it. I don't flirt and i still get accused of it. Like what does a little flirty conversation matter??? Im still coming home and f*cking Y0U at the end of the night.