That's not the problem, the issue is that he had to learn it from a random friend after he purposefully evaded the question.
The fact that he hid it intentionally mean he was either doing something wrong like cheating or doesn't think MC is an important enough relationship to discuss intimate topics.
Keeping up appearances is a shady excuse when you can just say "I'm already in a relationship, you don't know them, leave me alone".

Oh c'mon. Is MC being serious? The dude is gei and clearly closeted. He admitted to being gei and fucking other dudes, like I'm sure he doesn't want that girl cookie. Your bussy ain't gotta get all in a bunch just bc dude was keeping up appearances