Put yourself in their shoes and imagine you believed that terminating a pregnancy was morally equivalent to killing an adult person. Would “I don’t want it around” be a good enough reason to kill an adult? I’m guessing your answer is “no.” But if it’s not a good enough reason to kill an adult, and if terminating a pregnancy is morally equivalent to killing an adult, then “I don’t want it around” is also not a good enough reason to terminate a pregnancy. Obviously, you don’t believe that terminating a pregnancy is morally equivalent to killing an adult. Neither do I. But it’s pretty clear why someone who did believe that would insist on pregnancies being carried to term in most circumstances (maybe even all circumstances).