I think the comments are conflating romantic attraction and sex too much for this story. It's very unlikely Asumi would stop seeing the others, just like Mai isn't going to drop her job for her.
But romantically, Mai has always been the one Asumi was searching for.
It's unfortunate that multiple girls seem to have developed feelings for Asumi specifically, while there's been no indication of them developing similar feelings for anyone else. That makes it uncomfortably and disappointingly harem-like, since the story treats these girls as "romantically available to Asumi" without giving them romantic stories of their own. In that respect, Nanao got the better end of the deal : she moved on romantically and seems pretty happy being FWB.
What saved the story from that dynamic so far was Asumi's focus on Mai. It doesn't change the fact that the other characters were done dirty, but having a clear romantic objective made it easier to look past.
And I still think that's how it should, and hopefully will, end. If the story ends with multiple girls whose romantic lives revolve exclusively around Asumi, they're being treated less as independent characters and more as props for her romantic harem.
That's what makes Asumi's reaction to Mai's confession worrying.