I think the poster you are replying to is saying that rape is rape not because of laws, but because of the nature of the act. A 17 year old who has sex with their 18 year old partner, willingly, will most likely not refer to that act as "rape", even though by law the 18 year old has committed that crime. Had they waited a month, it wouldn't be a crime, even though it is doubtful how much more "able to consent" the 17 year old would have been in that scenario.