No it's illegal to murder and rape.
So the analogy
Most women go to get a job without intending to screw the company by getting pg immediately
Some women do
Most men who visit prostitutes aren't rapists/murderers (no stats there I have no idea TBH)
Some of them are
So your analogy compared the women who get pregnant to the men who attack women
So I don't think it quite works!
About this specific idea, I don't see how it could work in practice. I think your arguments are good - I think I think this is a good idea but it's not something I've ever thought about before!
I think it all depends on the stats doesn;t it? How much crime against women is actually being comitted by this group. If it is the case that visiting prostitutes means there is a 90% chance that you are a criminally nasty bastard, then there's a good argument. If it's only 5%, then not. IYSWIM.
In the cases where they take DNA to rule people out there are not usually objections, I see this in the same light. I think.
Dunno 
Certainly criminalising many things doesn't stop people doing them, but drives them underground, which would be the result of this if they tried to actually do it. Your comments remind me of the "soft" drugs debate - that loads of perfectly decent people are criminalised for doing something which (in their opinion) is pretty harmless. That is a good parallel maybe.
In practice it's an impossible Idea I think.
It would be interesting to find out some stats on all this but I guess that's pretty impossible.