Before fertilization, the sperm/egg only contain half of the chromosomes needed to become a human life, so I don’t think to call conception a starting point for human life is an unscientific view point at all.
Isn’t viability a pretty arbitrary line? And even if it’s the “right” one, some states allow late-term abortions once a fetus could survive outside of the womb. Do you believe that’s morally wrong?
Ultimately your argument of biological dependence being different from societal dependence comes down to your assumption that in society, there’s always someone next in line to take over responsibility of that newborn
What if there isn’t? Do you believe it’s wrong in that case? For example, imagine a new-born orphaned in a remote Himalayan village. Do the villagers have a right to terminate the new-born (assuming that they have adequate resources to do so)?