My boyfriend has this game and although at first cynical I did give it a go and found it quite addictive. I agree it's visually stunning and for the 1 week I was addicted to it (not played it in months now) I had a better virtual life than real life - rather than kill prostitutes (or anyone for that matter) or do the story lines I spent my time driving into the mountains, sailing boats, swimming, clothes shopping etc. You get the gist. The game is what you make of it.
If this game was about killing prostitutes or women I would absolutely agree with you but it;s not. Granted the storylines are based on crimes as the main characters are out to become rich and there is some violence involved. The characters have access to weapons (it is america after all) and it;s based in a city - just like in any city if i wanted i could go shopping or to a bar or i could walk round with a gun and kill people including prostitutes. I chose the former, if the latter is being done then it's generally the choice of the player.
Visiting a prostitute is the decision of the player, it wasn't part of any storyline i can remember (although granted i didn;t get very far). As the 3 main characters are male, 2 hetrosexual and the 3rd implied to be bisexual it stands to reason that the prostitute is likely to be female. Therefore if the player then decides to kill the prostitute it follows that they would be killing a female theyve had sex with rather than a male.
As has already been said however you can kill anyone you choose - it doesn't encourage you to kill prostitutes anymore than seeing a prostitute in real life encourages you to kill them - it's a choice.
For what it's worth my boyfriend played it for the storyline, I'd have been horrified if he spent his time virtually fucking prostitutes and killing them. There is a million things you can do with this game - it really is incredible, i;d be very concerned about someone who spent their time just fucking prostitutes and then killing them for fun.