Zizhen does know what's happening, and it's very much happening to him as well, and he should probably break away before this got very embarrassing but also... well... they were sort of courting, right? They wouldn't be kissing if they weren't, and even though he doesn't intend to do anything it was probably totally normal to be turned on by each other even before anything was official, because why would you only be turned on when it was appropriate?

He's a teenage boy living without the heavy restrictions of the Lan clan, he's dealt with inappropriate erections before.

Still, when he breaks away from the kiss to catch his breath, his cheeks are flushed and his smile is shy.