She isn't possessive, she understands Araragi very well, she has explained her actions quite well.
She is simply his girlfriend, and they both intend to get married someday, she simply guards her territory.
Senjougahara already said that she can't stop him from playing with other girls, and she tolerates it as long as this is the only thing, visiting friends and stuff. She is a tsundere, not Yandere. Now if she was a Yandere she wouldn't allow him to see other girls.