she is vying for his attention if she's clinging tht much and he really should take heed to what everyone is saying. I imagine he's not stupid and knows exactly what's going on, and may even unfortunately be enjoying the attention.

Don't think you're being petty or acting like a 12 yr old some women purposely go after men they like that they know are already attached, it's a game to them.

If he doesn't shun or pur her in her place I'd begin questioning his motives too as it would imply he's enjoying it and maybe even enjoying the jealousy. I'm not jumping to conclusions here I don't know the whole situation but if that's the case...

time to find a more loyal and understanding boyfriend.