Recently, a fellow pro-life atheist who is active in Secular Pro-Life shared this on facebook:
The problem is that many people throw this reasoning completely out the window when the topic turns to abortion. Here, I do not mean Penn Jillette; he's pro-choice, but has always been respectful of those who disagree. But for too many pro-choice atheists, abortion is "just a religious issue," pro-lifers are "the American Taliban," and any suggestion that abortion is unethical is just "fanatics trying to impose their religion on others."
I'm a fan of the "good without God" messaging that's been popularized in recent years. But that message is wildly undermined every time an atheist group advocates for abortion, especially the extreme position of abortion on demand at any point in pregnancy. Most people, whatever their religious background or lack thereof, find such a worldview morally abhorrent, for good reasons. It's enough to convince a person on the fence to stay in the atheist closet.
The pro-life movement and the secular movement have something in common: they are both in the midst of internal debates over whether or not they should be "single-issue." I have long encouraged pro-lifers to resist coupling abortion with issues like same-sex marriage or fiscal conservatism. To the secular movement, I now offer the same encouragement: keep it single-issue. Do not tie non-belief to support for abortion.