I thought we were talking about human beings?
Human beings are not magically made at conception/fertilization.
It is a long process to get from a fertilized egg to a human being.
A fertilized egg is not a human being. It can become one, but it is not at conception/fertilization.
Again you attempt to create an arbitrary disctinction without foundation.
Even your language is misleading. A egg after union of ovum and sperm is no longer an egg at all. It is inappropriate to call it one. The new life is a single celled embryo or if you want to be extremely technical a zygote.
We are talking about human beings. Early undeveloped human beings but human nevertheless. Humans go through many changes over the course of our lives. The embryo is different than the infant, the child, the adult, and the elderly and infirm. All stages are different but all are human and deserving of human rights and dignity.
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As for fetuses being somehow independent human life, well, they are part of a woman, so technically it is a woman life we are talking about.
At some point, the viability stage, you have to separate the two and say you have two human beings.
But at fertilization, on a petri dish? You are just irrational.
A fetus is certainly not independent it is a life that needs its mother for a time to survive a need that extends long after birth.
That is the primary reason why sexual activity outside of marriage is ethically problematic. It often brings new human life into the world. New human life that emerges into an environment that often does not welcome or support its existence. It is immoral to engage in activities that lead to such an outcome.