The assumption implicit in your argument is that the relationship is equal (as in the case of a boyfriend/girlfriend). I'm not sure that same assumption holds for a parent and a child, as much as the media tells you otherwise.
In an adult-adult relationship, the two parties should be equals, with similar cognitive and emotional maturity. They should treat each other as such. This isn't always the case. Even loving seemingly healthy relationships may involve some form of manipulative behavior on both sides.
Transgressions occur all the time, in all relationships, you have you to use judgement to figure out which ones get a pass, and which ones don't.