Zeke is indeed actively dating Miriam while still married to Keyla. If you are on Julia's path, it's put right out into the open in the story; which you don't see on any other path. Also on Julia's path, Miriam and Julia talk and Miriam airs some concerns she has that something is fishy with Zeke.... and it is.Zeke is his own person, who made his own choices and mistakes. Yeah, what Keyla and MC did was bad, but both him proposing marriage out of a fear of losing her, and Keyla accepting the proposal out of fear of being alone, play a much larger role in his current unhappiness.
I don't remember if it's outright stated that Zeke had his own extracurricular activities. He does cheat on Keyla in the present, as a result of his dead bedroom/marriage.
Sure, Keyla shouldn't have accepted the proposal... But I'm guessing that in the MC's mind, he did the right thing by breaking things off with Keyla, and didn't see any value in hurting Zeke even more with the truth. Though, I do agree that from a certain perspective, the MC is a special kind of asshole who's able to sleep with his best friend's partner while keeping that friendship up.
So I don't understand the crosshairs and anger solely on Keyla. Keyla went into the marriage for suboptimal reasons. Zeke did solely for his "desperation reason", that I presume will also suboptimal. Keyla in the past cheated on Zeke. Zeke is currently cheating on Keyla. Both are hiding it from each other. It seems like a 50/50 split for the hate/anger between the two of them right now. Both made bad decisions equally on this one...