I leave my husband to be with someone else. I leave my partner to be with someone else. My husband leaves me to be with someone else. My partner leaves me to be with someone else. Everyone knows about all about it, and sometimes, it feels fine, and other times it feels crappy.
Recently, I spent most of the day Saturday with my metamour, Camille, and my daughter at a women's self defense class. Russell dropped us off, and at the end of the day, picked us up. Sunday, Camille and I spent the afternoon together. Again, Russell dropped me off, and we joined him later in the day. He was noticeably a bit subdued that evening, and, when asked, said it felt strange to be leaving us all weekend. A bit of a downer really. That's part of how poly is different from monogamy though.
Sure, people come and go in mono relationships too, but they are usually leaving a partner behind to go to work, or some sort of activity that is less overtly personal. Bonding time, sex, intimacy, shared experiences, these things are the currency of relationships, and in poly, there is almost always someone who is being left out, or left behind.
Dealing with leaving someone you love behind, or being left, is part of the skill package we are all working on in our pod. It's always easier when everyone is feeling well-resourced, and has other options that they find valuable. It's harder on the weeks where everyone is stretched, and no one is feeling quite like they have "enough".
I'm in relationships with these people because I like being around them, and the whole "absence makes the heart grow fonder" trope doesn't really work for me, so this is an area I'm consistently working on.