I have met several pro-lifers who are not religious. In fact there is an organization called "Secular Pr-Life" that focuses on non-religious aspects of the pro-life movement. It is true that a majority of the pro-life movement is religious and many of the most vocal ones are Catholic but that hardly means that they all are and also proves there is more than just a religious objection to abortion.
Go to the Doubtingdave's DDO account, go to the big issues section and see his stance on abortion and then look at his religious views. He is a strong atheist but he is pro-life (he's against abortion with the exception of incest, rape and cases where the mother's life is at risk).
The fact is their are many definitions about when life begins, although one may be atheist, one may still hold that life begins at conception.
Although the religious do make up a sizable majority of the movement, not ALL pro-lifers are religious. This is a generalization and a logical fallacy to assert this.
Just because one values a life does not make one religious. I have met many who are pro-life stance but do not adhere to a specific religion. Many consider all life no matter the shape or form as precious and should be protected. Life is not strictly a religious factor, but a factor of the basic philosophy that we should all share.
Simply because someone is a pro-life advocate does not mean that they have to be religious. It depends on the individual's opinion about when a fetus truly becomes a human being, and because of this, can be based more in common human decency then in religious law. A person who is non-religious can still easily take the stance that developing fetus is a person and inherently has all the rights to life that a person deserves. Debates regarding abortion are not strictly religious.