deciding what others can or cannot do is a whole other moral discussion.

To answer your question. They consider the argument of “where do you draw the line” to be a red herring.

Consider the following: if a person is in need for a kidney transplant, or else he would die, would it be ethical to force someone to donate their kidney against their will? I think not.

Same applies to abortions. You are being forced to feed a parasitic being in your body, a being that destroys your body in the process. And not having an option to abort would be to take away your bodily autonomy.

As for the line, I think that the person making that choice is the one that draws that line. It is not for us to decide.