My stance is that we do not agree on the standards for "class, tact, and maturity," in this context. I haven't read anything to indicate that Fisher hasn't acted with such, other than Barnes' own hurt feelings, and vague comments from him that have implied that Fisher was in the wrong. Except that I don't consider Barnes to be a credible narrator. I think that he's exaggerating how things played out, if not outright lying through his teeth, in order to make himself come across as sympathetic.
Like I said before, I think that believing that Derek Fisher is the "bad guy" in this requires you to believe Matt Barnes' version of the story. And I don't; I find Barnes to be totally untrustworthy, and not credible. I do not think that it is inherently a violation of any "unwritten rules" to date the ex of your former co-worker. I think that you're taking Barnes' word for it that his relationship with Fisher was much closer than I believe it actually was. By all accounts that I've heard or read, Fisher and Govan have been kicking it for some time, and that Barnes already knew about it. I've read nothing that gives me any reason to believe that any of this was going on behind his back. Barnes himself hasn't even made that accusation, TTBOMK.
Govan and Barnes are not together. Fisher filed for divorce in March. He's not cheating on his wife, and Govan is not cheating on her husband. Other than expressing some vaguely patriarchal notions of 'ownership,' you haven't really articulated why you think that Fisher has acted dishonorably in this matter.
EDIT - Since mods can post in locked threads, I didn't realize that it had been locked until after I posted my response. Since I don't know why the thread was locked in the first place, and there's nothing about it in the mod forum, I don't really see why it needs to remain locked. I trust that the conversation will continue to be at least as civil as it has, so far.