@Lilliwyt "We were just playing pretend" - For Yoru it actually was. From the flashback itself it is hard to see what relationship they had.
a) A friends with benefit relationship that was continuous besides Yorus relationship with many boyfriends. Up until Sei decided to monopolize her
b) A relationship that only formed as a patch up / a sholder to cry, when Yorus straight relationship ended as comfort thingy between the relationships with boyfriends. But other than Yoru Sei didn't see herself as a filler e.g. sees the suffering of Yorus straight relationships and starts acting on it in a mindset that she would be a better partner than a male. And the sabotage came into being because Sei sensed that this relationship would have worked out.
c) An actual relationship where Yoru gave it a try and rejected Sei, as soon as the moment came of a new love with a men - that what we saw and lasted for 3 months until Sei interfered with it to monopolize her back into it.
And after that they parted and Yoru got married in the following time. In case "c" it was for sure pretend only to experience what it's like. In case a it might have been mutual and wasn't pretend but Yoru weren't sure, however got pushed away in finding an answer when Sei imposed to be number one on her. And in case b, well it was also pretend at least in Yoru's conscious. In the subconscious it might be different, but she never acted on it and drifted also appart.
So whatever the reading to this flashback is in the end Sei steped on the feelings of Yoru big time and Yoru parted because of it. Sei is just something else. Dunno don't stick your dick in crazy. But it is entertaining to read such a story non or less.