Is it your opinion that voting for Romney is voting pro-life? Not everyone holds that opinion.
A general election is a binary choice. In such a case, voting for the lesser of two evils is the morally correct choice.
Refusing to vote, or voting for someone who has no chance of winning is a narcissistic choice, for those who place self-centered preening about their own “purity” above voting for the best available real-world choice.