Because making respect contingent on "whether you agree with the issue or not" is a great way to make everything devolve into partisan tribes, eg. "anti-abortionists: why do baby killers deserve respect?"
It is when that issue is personal privacy and our fundamental rights. Why would I want to respect them when they want to further dehumanize us?
You really can't see why this can't be applied to abortion as well? I'm not even against abortion and I can see the problem with what's essentially "if you you disagree with them enough you don't have to respect them".