Responses
Im pretty sure if he so much as wrongly touch a hair on Rosa's body he'll have her mother's wrath to face... Also i think you're forgetting she is blackmailing him with the ring. He doesn't wanna loose the ring Rosa's mum bought him and Rosa is threatening to throw it away if he doesn't play along... So it's not as easy as you are making it seem

Yeon-Woo is a big guy... surely he can easily throw off Rosa who's smaller than him and resist her like he did in the past. Is he actually doing it on purpose? Pretending to be the weak and helpless one because he enjoys it and wants her to think she's got the upper hand to feed her sadistic ego?
She does have leverage on him and could easily out him to her mom that something happened between them (though he was unconscious back then)... But his desperation to not get kicked out must be sooo strong that he chose not to fight back.
IDK anymore... We're still barely getting tidbits of info with every chapter that comes out.