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

Because slavery is illegal.
Because he doesn't do 50% of the child bearing.
Because, if a man would want to have a kid without marrying a woman, or even caring for her, he would just have to impregnate her and wait for the baby-growing thing to produce.

When it becomes possible to transplant embreyos into men, they can bear their own. Will they?