You: Minding your own business, out with a date or a boyfriend
Him: Minding his own business and enjoying your company
Another woman (his ex-girlfriend, an overly friendly waitress, or some other miscellaneous female): Makes eye contact with only him, uses body language to shut you out, may 'accidentally' spill your drink, makes sweeping gestures with her eyeballs to suggest you're wearing hideous clothes. In extreme situations, she may mention that you're not very good-looking.
It still blows me away the way some females treat other females, yet twice in two days I've heard from women who've been harrassed by a rival for a dude's affection. If this happens to you, feel sorry for the poor idiot who thinks her life is going to improve by making a play for your date.
Whatever you do, don't challenge her. Avoid telling her to go to hell, smacking her in the head, or giving her the finger. Do not-- I repeat-- do not reduce yourself to her level. She's desperate. You're not. She's also a loser.
Blot your spilled drink. Pretend you didn't notice that the sorry creature suffers compulsions to poach other women's boyfriends to feel better about herself. Did I mention that she's a loser?
A worthwhile guy will respond to her with casual but friendly disinterest, and then you've beat her. A substandard guy will leap on her like a St. Bernard. Even if this happens, you've still beat her.
Offer the pathetic woman your seat. If he's foolish enough to return the attention of such a low caliber individual, tell her she's welcome to him.
Then do yourself a favor and hit the highway.