I can imagine your girlfriend liking the attention of guys that are not you. It gives her confidence and maybe she likes the thrill of seeing how someone reacted to her just approaching them and dancing for 20 seconds. Big whoop.
The secret to a good relationship is both partners being equally confident. Insecurities (like the ones youre demonstrating) are relationship poison.
Also, shes not taken. Shes in a relationship.
Nothing wrong with having boundaries as long as they're reasonable and clearly stated. Not wanting her to dance with random guys in that kind of setting isn't unreasonable. Having boundaries doesn't automatically make someone insecure.
lol'd@the bolded though. it's the same fuckin thing. don't be that tryhard progressive guy.