Hm, I was of the impression that Akira and Ayako had been occasional lovers after Ayako became a widow.
It has a lot to do with Akira's first appearance.
Remember, Asuka had finally confessed, and Ayako had accepted her feelings (more or less) and agreed to have romantic kisses and share a bed with Asuka. Things were looking good... and then, at the very last page of a chapter, Akira appeared for the first time: she was at the airport, freshly arrived to the city, and was calling Ayako to say: 'Hey, darling, I'm back in town. I'm going to your place. I'll be staying with you. I love you.'
That was screaming Zorra-chan.
That's how the "ex who returns" always appears for the first time. Always when things are going well for the MC couple. Always a dramatic appearance in the last page of a chapter, where she reveals she's back to rekindle the old affair. It's like a law of manga or something, hehe.
I as well was persuaded, after a first appearance like that, that Akira was an ex-lover of Ayako. And tbh the events from that point on pretty much confirmed it... (Like that scene where Ayako, knife in hand, tells Akira that it's ok to fuck her since Ayako is used to it and doesn't mind, but she better keep her cotton-picking hands off Asuka!)