SPOILERS!!!!
I'm not sure if this is true but I've seen spoilers where the bf is a scumbag who just wants to sleep with fl and wants her to rely on him but always gets rejected then he sleeps with the blonde lady.
But other than that you're right. Though fl is in a more shitty situation while her bf seems content to where he is. Also I hate these sort of mls i wish he suffers in this story.
Prob because she believes that the bf would jeopardize their chances to return (by displeasing/stoping the douche ml's assaults) by interfering/screwing the deal and I disagree it is completely reasonable and make total sense that she feels guilty. She's being coerced not forced (except for the first time), there's a thin line of difference between the two. For ex. if this was a noble girl webtoon with sparkles, flowers and pretty dresses, the heroine would never agree to what ml suggested... It's coercion but she agreed to the deal and that's what eating her conscience: she could have told the bf, left the mansion and found another way, she also feels guilty because the first time she did caved in to pleasure, + she's screwing the lord while keeping her bf in the dark .
The heroine is as you said graped. It is very VERY hard to say THAT happened to anyone. Even if it's the most efficient and logical action (seek help), most people feel an incredible and irrational shame telling others it happened.
SPOILERS
Just like what another person said. The ML of the story is the horrible lord. EVERY SINGLE media that have a male lead being this horrible usually gets an OK pass because the authors will intentionally make everybody else a bigger and trashy monster than him.
Btw I don't think ML told her it would take BOTH of them back so she could totally just be screwing her boyfriend over and keeping him in the dark for no good reason.
To be honest, that's how the author's often justify choosing the bad ML, that just turn the other love interest into a douche.
I feel so bad for the bf, he doesn't know that his gf is getting graped. She also isn't telling him cause she feels guilty which is why tf would you feel guilty about it when you never consent it? Damn I just feel bad for them, they're in a shitty situation they don't have the money and power to help themselves to get out of that place.