my (perhaps controversial) opinion is that post-s4 dean could rape sam even when he's not magically affected by anything. i'm a firm believer in the idea that there is nothing special about sexual violence. at the end of the day, a man who would beat you, lock you up, chase you down and ignore your boundaries in all other areas of life is absolutely capable of doing that in a sexual context as well.
personally, i do think moc!dean would probably have days where he'd be incredibly mean and violent in bed, but i also think sam is so disturbingly submissive in s10 that he'd pretty much never say no to anything. so while i think some of the sex would be painful (and at times even entirely unwanted), i also think sam's lack of boundaries would largely blur the lines between sexual abuse and "just" deeply unethical and un-negotiated kink.