Why are you opposed to abortion if it’s legal?
Just because something is legal doesn’t mean it’s right. At one time, slavery was legal in the United States. Slavery did not become wrong when it was abolished after the Civil War — slavery was always wrong. The injustice of slavery ended in large measure thanks to people like William Lloyd Garrison, Frederick Douglass and Harriet Tubman, who spoke out against the unjust laws that kept millions of black Americans enslaved.
Likewise, the pro-life movement is speaking out against the unjust — but legal — killing of unborn human beings.