That was just to set up the twist of Sayaka and Kasumi being in a relationship, nothing is implying either of them would be fine with multiple partners.
Oh I don't think Kasumi "would be fine with multiple partners," I think she specifically wants that for herself lol. That girl's secretly way more bold than anyone realizes, and that's been the theme of multiple reveals about her, right from her introduction where Michi thinks she's friendless only for Kasumi to reveal she's got a senpai bff with matching hair styles. Bet she ain't vanilla either lol.
Like yeah, sure, it's a crack theory basically. But for all the things I can explain away, there's one I can't: why did Kasumi come out to Michi because " I want you to be friends with Sayaka-senpai". She just watched a whole chapter of them getting along for her sake, so what role does she imagine being openly gay will play in that friendship?
It's fun to think about, if nothing else. :P