I totally get your point with the 3 and you are right but a straight man does not understand a gay man "reality" because he never experienced it so nuanced language will not even be thought of when trying to convey an idea. Straight is perceived as the norm. To people that are part of a "minority" group are sensitive to the words they use and perception of language because on a daily basis they are confronted with constantly defining themselves to others and apologizing for their existence. Straight men (and partially straight women) never had to deal with that because they are the norm so that instinct does not come second nature to them. This is why the context of who is making the comment is just as important sometimes as what is being said.
Whilst I do agree with you on why this happens and just about everything else you said. I think we can sum it up as blindess to privilege basically. The only thing is I still wouldn't call it an excuse. And this goes for anytime there is an offline/online inversion of reality when it comes to privilege (the thing is offline privilege begins after you click the x button, but online privilege is only there before), and you see people who don't experience certain things in their lives that other people go through, not bother to use this site as a resource for understanding those experiences. Then they get frustrated (I can see the frustration it's a human emotion if you feel under attack), but don't think well why not find out about these experiences before showing myself as blind to them unintentionally because he felt a single moment of discomfort.
So here of course, straight men in an environment with a disproportionately (compared to the general population) high number of gay/bi guys, then you should use the site as a tool for actually finding out about their experiences, and the way these differ. Then you might have known HOW you actually should write the post, and how you should communicate your frustration with us. This exact same scenario would have happened with any other "minority" dynamic we could name if the situation was comparable, whether that's white people doing that on a largely black website, men doing it on a largely female website, the list could go and on, and the reaction to the badly written post would have been the same.
It's not like any of us are saying, he's a bad person for not "getting it", but it doesn't look good for him if he's not bothered to try doing it in the last ten years. And I'm not talking about anything sexual there, but just try to understand what experiences we go through that differ from his, and a vague idea of how he should word his frustration. Because this has such I know it's an adult site with a penis to face ratio of 50:1, but that resource has been available to him from day one, just a shame he didn't use it. Imagine if he wrote this:
"Look guys, I do appreciate that you guys are constantly being assumed to be straight offline, and I do understand that this is the online world, not the offline world. I know I should have blocked him on kik, maybe I could have stated that explicitly too on the kik group. I get you go through different experiences than me, but there was a guy who wasn't taking no for an answer and wouldn't respect that I am actually straight. I do get he may get he may have come with a false impression because there are guys who aren't not being honest with themselves about their sexuality when they write 100% straight, I know you guys know they lurk in the chat rooms, but in my case it is true. Do you have any advice?"
I can't imagine that post ever receiving an adverse reaction, because.it showed a basic understanding and awareness of how experiences of the world differ. And plus a tiny bit of self-reflection of what it's like when the shoe is on the other foot (because it will be the only place he'll ever experience it) might just have given hiim an insight into what homophobia is like and what other men who are different from him go through.