You have to read between the lines to see it, the problem is that the characters are too similar to other stories so it is easy to relate them.
cheating isn't a theme at all
That does not mean that it has not happened in the life of one of the characters or that it will happen in the future.
When you read a lot Yuri you realize that the NTR is part of many of the lesbian relationships, due to the promiscuity and it is difficult not to see it even when you do not show it directly.
Yuri really PURE stories are very rare, almost non-existent.