It's not realistic, it's melodrama. It does not make any sense why Emi would accept the break up with a smile, it doesn't make sense that she doesn't say ask about her lesbian girlfriend deciding to go out with a man, it doesn't make sense that Mayu would just show up to hang out, it doesn't make sense that Emi would allow her without saying a word. Even in the most toxic relationships one person at always asks "why".
That's funny. I could assure that this is close to reality. This actually happened to me. Tho I am that shitty gf. I might have done something more horrible than what Mayu did. But I got another chance and I'm still enjoying it until now.
Liking or disliking something like this won't change the fact that story like this also happens in real life. Each people have their own stories. It's not just all about fluff and sweet stuff. I cannot change what already happened between us but at least we learned something from it and appreciate each other even more. We don't live in the past but rather use those experiences and move forward. Shit happens and it's really up to people how they will continue their story and I think that's what life is all about. That's reality.