So why can't he take that responsibility and be granted 50% of the say in whether the child is born or not?

Because that's not responsibility, seeing as how he won't have to experience fifty percent of the abortion. If he doesn't want to be a daddy, all he has to do is keep his little spermies to his little self.

BTW, my comments are not intended to address the tragic situation of the frozen embryos. I don't have any good answers for that one.