That happens when the author rushes to give Todou a girlfriend so early in the story.
Nah, it wasn't too early for two reasons. First, they don't have complete control over the length of the manga, so they didn't know for sure that it would still be going on 3 years later. and that chapter 29 could be considered early. Second, Itsumi's relationship with Rin is a big part of what enabled her growth in the last two arcs because she finally has secure love that her family wasn't providing. Itsumi would not be the same person without this relationship.
The girlfriend in question is forced to do and say things that she would not normally do and that are contrary to her nature.
The wording is important. Itsumi didn't say she turned down Kurohime because she loves Rin more but because she was already dating Rin. Saying it the way she did leaves room for doubt even if Itsumi didn't mean it that way. And Rin needs to crush those doubts even if the process of doing so is against her best interests.
Rin has done this before as seen in chapter 25 page 5. Itsumi was being wishy-washy about why she invited Rin over to spend the night and gave the impression who she picked wasn't that important. Rin was willing to go home and have Itsumi invite another girl to spend the night in her place and only stayed once Itsumi admitted she wanted to see Rin. Leaving over something like that to let someone spend the night their crush is not something most people would do since it is not in their best interest to let another person get close like that. Yet Rin was willing to do this. Rin is not perfect, as much as she is for Itsumi, and this self-sabotaging is one of them. So this is inline with her nature.