What in a yaoi manga would make you regret reading it?
It can by anything. Rape, cheating, blackmail, prostitution, tragedy, weak uke, cruel seme, annoying rival etc.
What I hate the most is when the seme cheats on the uke and the uke easily forgives him. I am also unable to stand it, when the seme hurts the uke by neglecting him or by prioritizing something/someone else over him.
I don't mind twisted or completely effed up plots or characters but I'm put off by when crap like that is belittled and justified in the story.
I only hate one manga. Just one.
This one:
http://www.mangago.me/read-manga/if_you_do_not_obey_me/ reply
