How I see it....
Well considering the lore the game has for vampires, it would kind of make sense that Kagome is a bit more accepting, as are some of the other vampires. Okami makes sense too considering. In the end, different cultures have different views on relationships, seems "mythkind" is of no exception.
I mean most vampires assume they will have to stick with women to avoid the risk of going crazy or ghoulification, so not really far fetched to believe some vampires would be fine with sharing a man. Kana and Kagome see the MC as completely different. This whole time, Kagome didn't even think she could be with the MC because of Kana and vampirism, now she sees it as possible and now she is going to think about it. Though it will still no doubt depend on Kana.
Poly relationships, harems, and thruples exist in the real world. If a prince from some shit hole country can have a harem, I am sure that a Demi-god that is immune to vampirism (probably won't need the meds) will have no trouble getting one.
Kana on the other hand, even if MC didn't have a harem and was simply came back into her life and was dating another girl, she'd still be hurt because of the expectations she built for herself. If MC just simply rejected her and there was no one else, she would still be devasted because of the pedestal she put him on. There was no way to avoid hurting her unless MC simply chose to be with her exclusively, but even then, even if he was 100% loyal, she would still be jealous and it would eat at her.