What is the lines of cheating in a relationship? Talking to other people of the different gender? Long eye contact? Physical contact? A hug? A kiss? One night stand? Frequent hook ups? So many people have different limits on what cheating is.
The truth is that sometimes... people cheat. I've done it. A few girls I have been with have done it with others (and once to me) and I didn't judge them. We had our reasons and not proud of it, but it happens. I have a gut feeling from hearing so many similar things from others that a majority of people have done so without even realising it or wanting to admit it.
At the time he was said to be flirting with other women he was not really well known. Not in any way in a position of power to abuse. So if his only sin is just being unfaithful in a relationship that ended anyway, then in my eyes it is nobody's business but the theirs.
Having said that... I am bias because I am a Sjin fan. I am glad it is being looked into, and if that was all there was to it (aka no underage stuff) then make as little information available to the public as possible to respect his private life.