Why isn't in-vitro considered morally unacceptable?

Those spare embryos are considered property. They can be stored/destroyed/used for research or donated to other couples. Pro-lifers find most of these points offensive. They're comfortable with natural loss of embryos which happens in a significant number of pregnancies - they can't not be, there's nothing we can do to prevent it - but planned destruction of what, to them, qualifies as life is intolerable. It depends on when you think life begins - if it starts at conception, then planned use of zygotes with no intention to attempt to save the potential fetus is clearly immoral. I'm not sure when life begins and I'm a doctor. Legally we're in a bit of a spot also as we're much better at saving lives of premature fetuses - can even maintain life at gestation age below the legal cut-off for abortion. The issue is - what qualifies as human life? When does a bundle of cells deserve the same rights we expect for ourselves? Hope this helps you see their perspective.