Strange, isn't it? I have asked myself that many times. The best I can come up with is that they see any created life as 'holy'. And somehow are able to not differentiate in that between creation by man and creation by their god. Then that means that you are not allowed to have an abortion, since then you 'kill' a life. Personally to me, it does make a difference if you have an abortion when they fetus is just a few cells, i.e. in the first few weeks. But not to them. To me it also is way more unnatural to be in a laboratory creating a human life and inserting it, than not wanting the potential life and deciding to get an abortion. But in the case of the people you mean, mostly it is based on religious beliefs and in the discussion, when is 'life' actually a 'life' and what is a 'life', created by men or nature.