It’s undeniably true that one does not have to be Catholic or even Christian to be pro-life. It is also equally true that one cannot be Catholic without being pro-life. To quote from the Catechism of the Catholic Church:
”Since the first century the Church has affirmed the moral evil of every procured abortion. This teaching has not changed and remains unchangeable. Direct abortion, that is to say, abortion willed either as an end or a means, is gravely contrary to the moral law… Formal cooperation in an abortion constitutes a grave offense. The Church attaches the canonical penalty of excommunication to this crime against human life. A person who procures a completed abortion incurs excommunication latae sententiae, ‘by the very commission of the offense.’”
Abortion is different from other sins because of its gravity. It is the most final and audacious act — the taking of a human life — committed against the smallest, weakest, and most completely innocent of humans, the unborn child.
For this reason, the Church hands down the serious judgment of excommunication for anyone who willingly participates in abortion. You can disagree with the Church on a whole host — pun intended — of issues, and still remain in communion with the Catholic Church. But if you have an abortion, or take someone to have an abortion, or encourage someone to have an abortion, you’re persona non grata as far as Rome is concerned.