Why do some pro-life people seem to think that babies created by rape or incest have less value than a baby created accidentally by two consenting adults?

I was idly following a pro-life debate on FB. The prolifers were so adamant that an abortion is murder at any stage of a pregnancy.

Then one of them smugly said, “However, I would allow an exception due to rape or incest.”

I responded with a short version of this question, and apparently the thread died out. Or some one aborted me. Who knows.

How can they justify that stance?