You can have sex with and murder prostitutes if you want to. You can also play through the game without ever doing those things. You can also have sex with prostitutes and not kill them afterwards. The choice is on the player. I don't use that feature but to me it's like the hunting of animals in Far Cry. I don't like those features but I like the overall game.
The thing is though, why is a side activity of one game enough to condemn it? I am not comfortable with the killing of prostitutes in game so I don't do that. But I'm also very aware that it's just a game. Games like movies often have themes I don't like. Games are often allowing the player to do things they would never do in real life. For example in the Sims I can play as a burglar and steal things from people's homes and community lots, I can be a marriage wrecker and pop out babies with multiple men whilst ignoring the needs of my existing children to the point they are are removed by social services. Sims can have sex or woohoo on a casual basis, they can even have clubs for that purpose should the player want too, hell you can even set up clubs with the some intention of fighting other people, if that happens to include women so be it.
I can kill children, adults and the elderly by locking them in a swimming pool and remove the ladder, I can become a gold digger and kill off my spouse once he's no longer useful and get all his or her money. Adults can attack children and let the fur fly. Children can shove their parents.
All of this is unmodded and all possible in one version or another. (As I said before there are some quite worrying mods out there and as adding mods is quite simple, I would be concerned if I had a child playing, to ensure they weren't modding it!)
And while that's very extreme behaviour and there is a lot more to the game than that, my point is it is possible and there's not many parents that object to it. Is it because graphically it doesn't look the same? Is it because there's no swearing? Is it because the violence is more comical and not as realistic as GTA?
Back in the original game, the developers encouraged players to get rid of unwanted sims by building a 2x2 pool, tell the sims to get in and then remove the ladder so the sims will drown. It's in the Guide. But that seems to be acceptable. Whereas one part of GTA is enough to condemn it even though it's so much more than that.