My theory is Akimi married Kitahara-sensei (because that seems to be his grown-up daughter talking to someone on the phone in the beginning) and I think they only struck a deal with each other so there's no feelings involved. I hope so anyway, loving this story so much so far.
BRO THEY SAID THEY WONT CHEAT SO WHY IS SHE MARRIED TO ANOTHER MAN WHAT HAPPENED I CANT I NEED TO KNOWWWWW