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.