Would it be worth reminding ourselves of what he actually said? Some of the comments here are straying away from that.
From BBC News:
When BBC interviewer Victoria Derbyshire told him "rape is rape", he replied: "No it's not, if an 18-year-old has sex with a 15-year-old and she's perfectly willing, that is rape because she is under age, she can't consent. What you and I are talking about is we are talking about a man forcibly having sex with a woman and she doesn't want to - a serious crime."
So the distinction he is trying to make is the one between (1) what Americans call 'statutory rape' ie it's defined as rape purely because of the age of the victim (who may or may not be willing) and (2) the kind of rape where the victim is an unwilling adult.
Of course he is wrong to imply that only the second kind of rape is a serious crime, but the distinction he's making is perfectly valid and (I assume) is the reason he took issue with the statement 'rape is rape' It's not about the level of violence or force involved, as some people here are suggesting.
Another point worth making is that the proposal to offer a 50% reduction in sentence in return for a guilty plea is not a completely new idea as defendants pleading guilty can already get a 33% reduction.