It did not occur to me until after several people pointed out that out that oh yeah, they presumably had sex right before this scene. That wasn't really my intention! In my head, this scene opens soon after Lanyon arrived home from his father's house, but looking at it now. Hmm. Yeah. He was definitely naked before this scene started.
So that's a little weird, but I can work with it! I feel like Lanyon got back to his dorm room and pretended like everything was normal, then waited for a quiet moment to casually mention the engagement so it wouldn't sound like a big deal or anything. He's like a cat who hides their injuries on instinct.