Because they were jerks. It's that simple. And your parents were jerks for allowing it. They were your GUARDIANS. A guardian guards against harm, both physical and emotional. I would have thrown those idiots out of my house if they treated my kids that way. Also, you mentioned that the situations were different when parents came over with their own kids. That tells me the ones who did not bring kids probably didn't have any and I hope they never do. You are not wrong to question their behavior. It was deplorable and you should know you were right to feel hurt by it.
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I'm not likely to be around much longer. As before when I first signed up here years ago, I'm finding that after a long hiatus, and after only a few days back on here, I'm spending way too much time here again already. So I'm requesting my account be locked, banned or whatever. It's just time. Until then, well, I dunno...