the magic system in avatar isn't very good, the rules aren't laid out very well and often don't make sense. cum is mostly water, so in theory you should be able to move it, but i know that blood bending is a skill in this universe and not everyone can do it, it's like super special thing passed through genetics and forbidden. but it doesn't make any sense, because blood is just water so any water bender should be able to do it. some magic systems like Mistborn at least lay it out cleanly, metals inside the body can't be moved, why? no one knows, but at least they make it explicit so you can't poke holes in the world building.