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I just recently was in Japan so I can see some houses. And yes, they don't have a lock on any doors except the bath (not always) and the main door.
It's a tatami room with a slide door, and usually a house with tatami room is pretty big so i guess people wouldn't really be able to hear him. And everyone pretty much asleep anyway.
Do Japanese homes have locks for their rooms or not?? forget yaoi part... man...his parents are also living in that house ...what if his mom comes in suddenly!!! ┑(￣Д ￣)┍