I think it's mostly because a big deal has been made out of keeping her virginal throughout the seasons (she kept being rescued from rape and Tyrion didn't even attempt it). I believe people see her, on some irrational level, as "ruined" now, their fantasy of her remaining a virgin until she gives herself to some beautiful kind prince has been shattered. While I think that's fucked up on an entirely different level, I can't think of any other explanation. I thought Dany/Drogo thing was much more disturbing and disgusting since he rapes her until she gets Stockholm syndrome and then all of a sudden we're supposed to buy it as some big love story (and people generally did). Now THAT is sick.
As far as this episode goes....I hope it doesn't mean we'll be losing Cersei :/