To K^M, it depends on their relationship entirely. I had a friend who fake flirted with tons of girls, but there was 0 meaning behind it. He was always being a goof. On the other hand, some guys do that pretending to joke, but have ulterior motives. You know the guy and their friendship, so you could only rely on your own judgment.
As far as cheating, I personally don't give a shit if someone's in a relationship if I don't know the other guy. If I know him and have any respect at all, I wouldn't do anything. I also wouldn't ever pursue a girl in a relationship. But if a girl is giving me signals, I'm gonna act on that. It's her job to be faithful, not mine.
I had a somewhat similar situation in reverse when I was dating the 36 year old in Korea. We were part of this outdoors group and both knew the leader well. She went on a small group trip with him and 2 other girls and he ended up trying to get with her. He knew she had a boyfriend, but didn't know it was me. She told me about it later and I thought long and hard about it, but I couldn't really be angry because he wasn't disrespecting me, he was disrespecting some unknown faceless guy. Had he known it was me, on the other hand, I would have reacted differently. He learned it was me eventually, acted like a child about it (teasing her for cradle robbing, etc). He was quiet for a few months and being awkward to me, then offered me some discount on a ski trip he was leading. I took that as his awkward apology and we ended up skiing together all day. I wanted to ask him about it, but decided against it considering his character (kinda passive aggressive, timid, awkward). We're all human and are controlled by our dicks.
Of course the same guy later fucked me over on a deal we had set up where I'd lead trips in his absence in exchange for free skiing, so it turns out he was a dickhead after all
A female friend of mine once caught her boyfriend responding to online dating ads, with all sorts of sordid flirting and whatnot, although he hadn't actually agreed to meet anyone. She forgave him and they're still together (this was around 5 years ago) but that probably would have been the end of it for me.
That's never innocent. One of the girls I met on Korean cupid told me after like a week of talking (and we both really liked each other) that she had a bf. She said she gets on there because she's bored. Yea right. She still wanted to meet though because "we have a connection" or whatever bullshit she said. I immediately threw out any desire to date her, but as I said if a girl is offering herself, I don't give a shit about her relationship status. We get dinner, go to a bar, then like 3 hours after I met her she says "want to go to DVD room?" Biggest clitoris I've ever seen