Locker room humor generally refers to talk between guys, which could have sexual undertones, but isn't normally something I'd think of as "sexually-oriented".
And flirting can range all the way from smiling long at someone at a cafe or calling them 'cutie' in conversation, to me spanking my s.o. as they walk by in a sexy outfit and telling them they're gonna get punished if they keep distracting me from work- so there's a huge range in there, some of which I'd definitely consider sexting, if texted to someone.
Frankly, I have zero sympathy for him, because it's very easy not to interact over direct message with fans at all, much less underage ones.
I've worked customer-interfacing jobs that required a high level of direct, personal relationship-building before (sometimes even gasp with people I found attractive!), and I never once felt compelled to take those communications into a private space, and there was never even a potential for those people to have been kids.
You don't "stumble into" private messages with a minor that "get out of hand".