I'm rather upset the other thread was locked so I'll comment on the Reina situation here.
I think Reina's insecurities are completely well founded. I would also be upset if a long term girlfriend confided more in a friend than in me, regardless of gender or attractiveness. It's a red flag for the relationship, since it shows you're not being considered trustworthy or reliable. The fact that Ayaka is an attractive girl could make things more pronounced, but I reckon that the overall problem is not even grounded in that fact.
Having said that, Reina's actions are completely inexcusable. Instead of communicating her insecurities, she:
1) bailed midway through a date after they hadn't seen each other for a long time
2) ghosted him for a couple of days
3) trusted a random guy's outsider's perspective about her boyfriend over her own boyfriend
4) decided to try "an experiment" and reciprocate said guy's advance (which in this case stopped at holding hands intimately)
She's absolutely terrible and stupid.
I understand that voicing out the fact that you're insecure about your partner's friends is very difficult and that it might even lead to break ups. It has literally happened to me. But breaking up over a discussion about your partner's friends is a million times better than cheating on them in retaliation or to prove a point, whether physically or emotionally.
Lastly, I do not understand how Reina had the gall to show up at the guy's apartment with a smile later on (when he broke up with her). Even if she didn't know she had been seen holding hands with that guy, she should have gone there with a serious face to apologize about her behavior during the date and the days that followed, and have a heart to heart interaction regarding the Ayaka issue once and for all. Even without the cheating, it wouldn't have been weird if the relationship ended on that day.