There are really only two big religions that are in a position to compete for global supremacy, which are Christianity and Islam. Religions can expand by conquest and conversion, but they also can also expand by reproduction. If Christian families have lots of Christian babies, then Christianity is expanding. If Christianity can expand faster than Islam, then Christianity wins.

Thus, anything which might lead to less babies being born, such as birth control, abortion, or homosexuality, is seen to be an impediment to the expansion of Christianity and its ultimate goal of global supremacy. That is the logical, hidden objective.

However, that is not the whole explanation. Christians have purely religious explanations for their opposition to abortion, they may believe that all human life is a gift from God and is sacred, that babies or even fetuses have souls, that even unwanted pregnancies are part of God's mysterious plans, and so forth. That is also part of their thinking. That prevents them from seeing a fetus the way I see it, as merely a potential person rather than an actual person.