I don't remember how accurate this is. Did Zeke cheat on Keyla in college, or was this just a suspicion? And I don't remember anything about him suspecting her cheating on him. That said, I agree that this leap in not-logic is a big ask. If Zeke had access to pussy galore, then something about Keyla would've been special to him, but not special enough to not cheat on her. I can understand Keyla's perspective better, agreeing to Zeke's proposal after getting dumped by the MC, maybe out of fear of being alone.
My general point is that young people are dumb, and rarely think things through. Zeke might have had a subjectively compelling reason to propose to Keyla which seem too farfetched from an objective outside perspective.