I don't think the "my body, my decision" argument holds any weight, because it's not "your body". Ignoring whether or not it's "alive", it has it's own DNA. It's not "your body". If it was "your body", then it would share your DNA. It's a mix of your genetic information and the genetic information from someone else.
The bacteria and viruses who live inside you have their own DNA. They are not part of your body.
Do you feel you have a right to take antibiotics, have a shower or brush your teeth, despite the fact doing so will end astronomical numbers of lives? Is it really your body?
I'm not religious, but life beginning at conception remains the only clear answer to the question of "where does life begin".
Do we need a clear answer?
See, what bothers me about that question is that it's a metaphysical question disguised as a scientific question. Life as a biological concept is a set of metabolic processes which occur within cells. Those cells, however, don't care about the body they are part of. They don't have intelligence or purpose. Our lives as multicellular organisms aren't a single biological process but rather the cumulative action of trillions of cells. Those cells themselves are also constantly dying and being replaced. We continue to exist because we aren't actually those cells, we are a pattern created from the interaction between them.
A fertilized zygote is not more complex or made of different stuff than any of the skin cells you shed today. As that zygote divides and becomes a multicellular being, it slowly develops its own pattern. That emergence of a complex living thing from the simple behaviour of mindless cells is gradual and incremental, and there will never be a clear line where it "begins".
But that alone is not satisfying to us, because we're caught in this weird and immanent experience of actually
being a pattern that can think. So it makes sense that when we say life we don't mean the life of an individual cell, we mean life as the stable continuity of existence that we experience. If you believe that that continuity begins at conception, then that's cool, but again, it's a metaphysical claim. We can't actually know.