I grew up in the 60's and 70's. "Mentally not all there" was a polite term that was used to describe learning disabilities, alongside more offensive slurs.
You say that in your definition, it means people "lacking in morality, compassion and decency." However, I don't see these qualities as linked to mental functioning at all. All three are related to value judgements, not mental capacity, and all are highly subjective. Compassion, morality and decency mean different things to different people. They are not absolute concepts.
The point is though, as Bertie's post illustrates, many of the men who carry out acts that meet the legal definition of rape see themselves as moral, compassionate and decent people. We see this in the way rape cases are reported in the media, discussions around the water coolers at work and the school gates and in internet forums. Apologies and excuses for acts that meet the legal definition of rape abound. That is evidence of a "rape culture," whether we like that term or not.
There are loads of studies about attitudes to sexual abuse, like the one Bertie Botts cites, which continue to show that like it or not, there are alot of men and boys out there who would force a woman (or other man) to have sex against their will if they thought they could get away with it. Odds are that they probably will get away with it.
That doesn't mean by a long chalk that all men are rapists. That just means that men who choose to rape can probably do this with impunity by convincing themselves and others that what they are doing isn't wrong. Imho, they are aided and abetted in their actions by the sexist jokes, anecdotes, sexist media portrayals, images in porn, automatic pity for the accused, victim blaming beliefs and righteous indignation of the wronged males.