Firstly, right off the bat, jealousy which is consistent and unjustified is bad in any relationship. Okay? Is that clear? That's bad. No. NO! HOT! BAD! NO! Okay, Readers?
Moving right along.
When a lady gets full tilt hit on by a man who is not the man she's been seeing for a while now, that lady would feel little flutters of joy if the man she IS seeing was at least a little bit, "WTF?" about it. For example - and this is completely hypothetical and definitely never happened to me in real life just last weekend - if I'm hanging out with this guy who's whatever and we've been seeing each other for a little while and some other dude is talking to me all fucking night long and then has his arm around me and THEN goes so far as to tell me he'd like to fucking "smooch" me (and that's totally a word I just pulled out of nowhere and no one has ever said that to me in real life just last weekend) RIGHT IN FRONT OF THE DUDE I AM THERE WITH, then that would be an ideal time for the dude I'm sort-of-seeing-or-whatever to be a little bit "WTF??" about it. Like, even if the situation genuinely doesn't bother him, he should at least fake it. Because that kind of jealousy makes my knees feel quivery. It's like swoon material from the Victorian age. I don't need a full on duel. I don't even need words exchanged. Just do something that says, loudly, "She is here with me. I like her. And I'll not be threatened by you, good sir." A lady likes this kind of thing, you guys. Let a betch know she's worth getting a little worked up over. Odds are pretty good you'll get laid.