I used to troll pro-choice people on Twitter by telling them I agree with them because it kills so many black babies and keeps the black population in check.
No matter how that reacted, I'd just ask why it matters when we both agree on the principal subject. Then I'd suggest we should work together to make sure abortion was more easily available.
They literally didn't know what to do. They didn't know how to argue against someone who was agreeing with them.