How many times does the author need to show that the MC and Itsuki are often together, talking about serious things, and we as readers don't get to see the conversation? And STILL people think this is some kind of mystery.
Itsuki and MC go to college, AWAY from the other girls, and afterward get married. The other four are just in teen infatuation, and it will be over him in a week of separation. There's nothing to feel sorry for. Teens in love is hardly a serious matter.