That’s my preferred mode of toxicity. It’s easier to understand how the MC can be drawn into their trap that way. If they’re just nasty and overtly abusive all the time, then why the hell would my MC give them the time of day (unless the MC is batshit crazy, too)? Now I want to check those out…
One-sided romances are always toxic, to some degree. Sadly, most of the romances in IF feel that way to me, because my MC is always the one having to chase them down to beg for attention instead of them putting in effort without being poked repeatedly with a cattle prod.
Then why is he a romance option? Because this, to me, boils down to “Ferrado doesn’t want the MC” and nothing can change that because of the master/slave situation. Don’t advertise a NPC as a romance option if they are not a romance option. Either the NPC wants to be with the MC–and will actually either accept the MC’s affections or take steps to show the MC their own affection (like that ever happens in IF, lol)–or not. In Ferrado’s case, it’s not. So making him a RO is either false advertising or a tease. Either way, the result is the same.
I mean, if Ferrado was ace, it’d be a different story. Then, he could say he’s in love with the MC but doesn’t have sex with anyone, so turning the MC down for that reason is a perfectly understandable character-based decision. Instead, it’s mealy-mouthed excuses about power imbalances that, ultimately, equate to Ferrado not wanting the MC. It’s just that simple to me.
For M, the same really applies. I’m sorry, but one drink does not alter my ability to make a decision (unless it’s like a 190 Octane from Fat Tuesday or a monster margarita made by someone who stubbed their toe when pouring the tequila), and treating someone as though it does–when they are obviously perfectly coherent–is disrespectful. I think this was more an author thing than a character thing, given the overabundance of “consent” moments there are in TWC romances. Musn’t touch your fuckbuddy’s elbow even when they’re clearly horny without a nod of consent first! 
Because he doesn’t want the MC, that’s why. It’s all excuses. It’s more of the one-sided romances, that are far more toxic than someone who shows a bit of jealousy on occasion.