Responses
I think in this one and Sweet Man, it can be explained that they are childhood friends or best friends, therefore already shared a bond before they did it. It is not hard to give your closest friend the benefit of doubt that they are not going to harm you. It is more realistic than Bella Swan who only know the dude recently.
my only issue with the plot is it's way to fast and Mitsuru's acceptance of just being drop fed information by a friend who's essentially lied to him for so long just culminates in them immediately having sex because that somehow just makes sense? It's the exact same problem I had with the plot 'Sweet Man' like your ignorance is constantly being taken advantage of by your love interest because you don't know what your agreeing to or being influenced into agreeing to, but that's seen as just perfectly okay because things just happen to work out? It just breaks the immersion when the characterization has zero self preservation or skepticism just it would be a hinderance to the romance. Like who thinks this way except Bella Swan?