A guy that has been pursuing me for over a month entertained another girl at my social event?

Long story short - this guy has been orbiting since a month or so trying to get my attention and to consistently spend time with me. I am very busy and we rarely have these moment of deep talk. But it happens. Yesterday, I had a social night I was throwing, he helped with groceries and cooking. He sat next to me, and we had lots of banter with the friends we have. We were very close and our knees were touching. And I saw this random girl from another house that came in and she was observing us the whole night. She finally got her moment when she inserted herself in our conversation. She started talking nonsense and as he kept replying, I kinda turned away and shifted entirely. While he went to wash the dishes, she stood next to him wiping. I left the room because tbh after the wonderful night, this wasn’t the cherry on the cake I was expecting. After that, I saw she lingered until the end. I came back to thank everyone else and he followed me to the opening of my room where I stopped him from entering. Said bye and that was it. I don't know to me it was just a major turn off. I don’t get it.
A guy that has been pursuing me for over a month entertained another girl at my social event?
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