She feels as if she has no choice but to accept his advances (which he hasn't made), because she has no money, no allies, and nowhere else to go; she believes that if he decides to use her like a fuck doll and then lock her in a servant's room until he wants another go, she can't complain or object.
Nuances be dammed, SEGGS, BRO! SEGGS GONNA HAPPEN! FUCK YEAH!
Okay, joking aside, totally with you on this. She misunderstood the MC's request to take a bath as "Get ready to have sex," which is why she's acting this way. And, I mean, who can blame her?
She was brought down to her lowest, stripped of almost everything, and exiled to the boonies to marry a guy with a reputation. Sure, she was a villainess, so she’s not blameless, but as the MC pointed out, she's still just a 15-year-old with in-game punishments that were so extreme it felt like the devs had a personal grudge.
No wonder she misunderstood the situation; she’s probably feeling hopeless, scared that worse could happen if she doesn’t go along, and has nothing else left. I expect the MC to be a decent human being and reject her advances.