Pretty much because pro-lifers are the ones who argue against abortion on the grounds that people who want to abort should actually just adopt out their babies. If you stopped using that argument, then you wouldn't be expected to be someone who might adopt a baby that might otherwise have been aborted.

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We've got rules and maps and guns in our backs, but we still can't just behave ourselves. Even if to save our own lives, we are a brutal kind. The Shins