The first time they had sex Cain more or less DECIDED they were going to have sex and didn't ask Abel's permission at all before getting rough with him. I would call that dubcon at the very least, but let me stop before this gets into an unhelpful "what counts as rape?" discussion.

Maybe Rape As Love was an extreme label for it, but for a comic where you might be expecting a loving relationship, all I can see is Abel confusing his lust for someone who abuses him, and that's what I had a problem with.