I have noticed this too and practice similarly to the two of you. I would never feel comfortable with a lover that doesn't consult my feelings before engaging in a sexual and/or intimate relationship with another. It just wouldn't work for me.
My feeling is that people such as that are unable to love deeply and respect the connections they create with others,... but I have no idea on this and it is simply my own perspective that keeps me from feeling hurt by such actions. I recognize that and live with the knowledge that it infuriates others. So be it. I think it's dangerous and uncaring and not my way of being poly. Seeing as many people as one likes without having a need to tell them what you are doing, to me, is dating.
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