Actually, I think Izumi cut Marin off because what she was about to say would have been an unintended insult towards Mizuki. "Hitting on" tends to be done by "bad boys" when you factor in cultural context. That or just plain interrupting being rude.Izumi is so considerate. Marin was about to basically yell "you're in love, moron", but Izumi cut her off and let Mizuki talk herself through it. That's an important thing to do sometimes. When it comes to complex personal situations, people rarely actually want to be told the answer. They want to form a dialogue about it and then come to the answer themselves. Many people will actually instinctively reject an answer provided for them by others, especially if it is provided right off the bat like that.