OP, you changed your story.
In the beginning, you said:
"Basically there's this guy I've been fucking for a few years now while he was on and off with his boyfriend. Supposedly they've been "on" for a year now but he would call me over while his boyfriend was away
Anyway, he just pissed me off and I'm at a point now where I'm very tempted to DM his bf on Instagram with all of the proof that I have of him being cheated on. I know that it's petty but I want to hear from others how petty they think it is before I do it"
Then, in a follow up post you said this:
"It is shitty but it's also something I wasn't even aware of until much later... in fact that's kinda the reason why I'm pissed"
You were fully aware that he was "on" with his boyfriend while you were fucking him. So what changed to piss you off?
My guess, and this is only a guess, he decided that he wanted to be faithful to his boyfriend and is breaking it off with you. That's my guess on why you are pissed off. So, following that logic, you want to get back at him by telling the entire world, via the boyfriends instagram and pictures (I'm presuming) that you've been fucking his boyfriend.
Basically, "If I can't have you, I'll make sure HE can't have you, too!"
It's a crap move on your part and shows the world just how untrustworthy and unhonorable you are.
However, as someone who has been cheated on, I would rather know that the man whom is supposed to be monogamous to me, isn't. Regardless of where the information is coming from, as long as it's credible information.
Take this as you will. You've already demonstrated your talent as a home wrecker.
Sklar