I think you're missing the point. I stick my penis in an animal. That is illegal. I stick something else in there to get it pregnant. Not illegal.
So this isn't about consent. And it's not about the laws lagging. It's about the law differing depending on why you're doing the thing to the animal.
I don't doubt that some vegans might treat animal breeding as rape but I don't think the general population does, or consider that element suffering in any way.