There was a discussion going on recently regarding the idea that one partner would be turned-on erotically by their partner's outside sexual activities. Let's call that "erotic compersion".
One point that puzzled me was the concept that this would create a non-consensual/undesirable sexual dynamic between the metamours. Now, if there was an expectation of sexual contact between the non-dating partners, I am completely on board with this concern! For example: Let's say that I am fortunate enough to make a wonderful erotic connection with another woman, and S or D were expecting to be able to watch or participate while she and I were being intimate physically. Um, no. This isn't a package deal, at least not unless it's specifically negotiated as such.
Let's look at another possibility: I'm happily engaged in an erotic relationship with another woman. S is working downstairs while I'm playing privately upstairs, and the happy noises emanating are turning him on. Is that involving him in the sexual dynamic I share with my female partner? Don't think so. Is it okay that I feel sparkly and hot when D gets home from a date with someone else? Is that me having non-consensual sexual feelings about them? I beg to differ. I'm getting sparked up about my partner being with someone else, not about the metamour.
Now, I understand that there is a statistically significant portion of poly folks out there that, while perfectly okay with their partner having other romantic/sexual connections, prefer to keep their partners and their activities entirely separate from the direct relationship shared. My approach probably isn't going to work with these peeps. Ditto for me with theirs. One of the things I like best about poly is the connectivity between the different relationships we all have, including the part where I get turned on by their outside sexual activities. Erotic compersion is something I wouldn't want to do without in my relationships.