Kind of makes you wish for auto-closing fence gates, doesn't it? I don't know if pressure plates would work for them, but at the same time, I find pressure plates to look out of place sometimes because you can't make them out of wool and use the same colour as the environment around it. Kind of also makes you wish you could tag animals as a way to keep track of them. At least with sheep, you can kind of keep track of them due to colour.