What I think is that Kouichi's trigger is more or less a deep desire to fit in with them (something like Jealousy), but there's hesitation as well because he has been living a boy for a long time now, and the transformation happened late. It's jarring for him.
What makes it worse is that he doesn't have that much time before one of them becomes his permanent figure. If it that happened earlier or the fix happens a lot later he would have a lot of time to decide. But then again, he has been living male his entire life until now so it's more natural for him to choose that.
It doesn't help that the others won't let him be. My boy- er girl doesn't have peace when it happens lol.