What’s up, man? I am a liberal that supports the right to choose, and I’ll answer your question.
The issue isn’t when life begins—sperm is alive, for example, and you can spank off into a toilet and flush that shit as much as you want and no one cares. The issue is a Constitutional one.
The Due Process Clause says, “No person may be deprived of life, liberty, or property without due process of law.” (Underline is mine.) So the issue is when personhood begins. Is a fetus a person, in the sense that the Constitution means it, or not? If it is, abortion is unconstitutional. You couldn’t just take its life because you choose to. If it isn’t, I and many others would argue that the government can’t take away a woman’s liberty to make that decision without due process. After all, we know that she is a person in the constitutional sense. Does that help at all?