I think that it's more irresponsible to have a kid that has to suffer from the mistakes of the parents.
I don't think anyone is arguing someone should be forced to give birth to a child they don't want. But, I think most people (pro choice/life) find abortion as birth control, for lack of a better word, immoral. People who fight for abortion rights don't do it so that people can have sloopy sex and be irresponsible and ultimately, those who use abortion as birth control hurt the pro-choice argument.