FORGET when life begins. It's a red herring. A distraction causing us to diverge rather than agree.
Right now, today, there are countless people needing blood transfusions; but I'm not legally forced to donate my blood to them (I should; it would be great; it would be the right thing to do; but I'm not legally forced to do so).
Right now, today, there are people needing bone marrow or organs; or would like eggs or sperm so they can have a child; or long hair for wigs for cancer-struck; but I'm not legally forced to donate them to these clearly worthy causes.
Lest somebody tries to muddle the issue, I'm also not required to donate blood or organs to my two children, or my sibling, or my spouse; I should, and I likely would, but I'm not legally forced.
.... So even IF we grant that a gamete or zygote or 4 cells or 6 weeks old fetus or whatever are live beings based on whatever definition we agree or fail to agree on; what rational differentiator would cause us to force a woman to donate blood and organs, sacrifice and risk their health for them?
I may have come to this point late, this may be an old perspective to others, but it helped me shortcut a tremendous number of complex and relative arguments, and go back to - "it's a woman's body; where do we get off forcing her to donate it to anybody else?"
At the same time, I must admit I now find it harder to understand and condone the opposing perspective; I see it as forced organ donation for a select subgroup of humanity, and while I'm a moral relativist, that doesn't mean that I don't find some things morally wrong :-/