Ciro Santilli is for abortion rights of women, until very late in pregnancy.
But it's not something that he would do himself, unless under extreme cases.
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First let's discuss the argument against abortion. Most people would save the life of an infant over that of an animal, even if the animal is more intelligent. The only unbiased reason to do this is that given nutrients, the baby will develop into a more intelligent lifeform in the future. Thus wanting to save a "clump of cells" isn't as irrational as it may seem.
But our hypothetical above assumes that both the baby and the animal want to live. By not killing themselves, all conscious creatures have implicitly expressed their desire to continue living, and most people agree that that is to be respected, at least for humans. Which is why killing a person painlessly while they're unconscious is still considered wrong. But this desire does not apply to fetuses before the third trimester, which have never been conscious. This is the moral argument for abortion. There is also the obvious utilitarian argument for it.