2%-16% of the male population should be convicted of a serious criminal act and put in prison for years at a time?
That sounds expensive.
In addition to what the professor said, the underlying assumption there seems to be that more stringent applying the law would have no effect on the crime rate. That doesn't seem to be the case.
In fact, some would even argue that the probability of being caught is actually an even bigger factor than exactly how big the punishment is. And as an anecdotal example, see the middle ages and renaissance, where disproportionately higher punishments and a disproportionately higher probability to evade the law, well, meant a lot of crime. As we had an increasingly good police and justice system, it didn't end up meaning we put half the population in jail for theft and murder, but actually that the incidence and prevalence of those crimes dropped.
And even for rape, _something_ must be having an effect on incidence rates, since they dropped 65% since 1993 and 6 _times_ since 1980. (So, yes, if you took the same poll in 20 years, you'll probably get a lot less than 1 in 6 raped.) There is a compelling case for the existence of internet porn as a factor, as supposedly now a lot more people are doing that instead of thinking they must get real pussy at all cost, but at any rate the correlation between internet access and the drop in rape incidents reported is pretty hard to dismiss. But equally a case is being made by many for it basically being more reported these days, and society being less inclined to blame the victim (not I said "less" not "not at all"), so basically the potential perps feel less like they could get away with it.
So basically it seems to me like a more realistic expectation would be that _if_ we could somehow get better at it, the result wouldn't be putting 2-16% in jail, but rather maybe putting 1% in jail and 1-15% getting the idea to find something else to do. Of course, it's _hard_ to get better at it without erring on the other side, so I'm not particularly blaming the justice system or anything, but just saying.