PackRat, our new friend tuxic is right. lxdm in Debian sid doesn't have many exotic dependencies. Why then installing lxdm bringing lxde+openbox in your case? It is because either an apt-get install alias or your apt preference is pulling in recommended packages. Create '/etc/apt/apt.conf' file and add following to it,
APT::Install-Recommends "0";This will prevent apt-get from installing recommended and suggested packages.