Well, Yokoi is right, it's not the first time Saitou was acting jealous, the first and the hardest was in the hospital when they visited Yokoi coworker.
The issue is Saitou clinging to Yokoi as her saviour and anyone getting closer would face an immediate response, be it verbal or even physical.
Although Saitou wasn't angry when Takamatsu offered to have sex with her to ease her pain, but she recognised that her approach was due to her own issues not romantic.
And we were supposed to make good memories couple chapters ago... damn...