I have trouble getting the proportions right, so when I'm done it looks unnatural, and therefore robotic, if that makes any sense. It's worse when I'm trying to draw things standing still. You can fudge more when it's a dynamic pose XD
Nope. It takes way more than straight lines to make a drawing look "robotic". In fact, most of the most skilled artists (the man who did all of the backgrounds in the movie Spirited Away, for example) use ONLY straight lines when they start a drawing, only to add minimal curves later.
Anatomy is more about how you draw, and less about what kind of lines you use.