If the man didn't want the baby, then surely just like a women can choose to have an abortion the man can choose not to be responsible by paying a set fee. If we do not allow men the right to abstain from legal obligations, then we have to also give the man the right to not allow his partner to get an abortion if he wants the baby and she does not. Rights work both ways.
Assuming the circumstances are those outside of rape, sperm donations, or something of the sort; one should take responsibility for his actions. If the child is to become a legal citizen (which it will), those who created that legal citizen should both be legally responsible for that person.
Although I would prefer one not being legally obligated to be, that simply isn't fair to the mother - as goes vice versa.