It must be provable beyond a reasonable doubt that the woman decieved the man into thinking she was on birth control, then this would be vaild. Thouhg if it was not brought up, or the woman says she is on birth control, the man should be able to opt out of child support
the actions of the man should take this situation into consideration, firstly by assess whether the birth control was applied competently and faithfully and secondly the consequences if they were not. This foresight makes the offspring his responsibility. (Moral/ethical argument vs legal)
No contraception is absolutely 100% effective. Both parties must recognize the inherent chance of pregnancy and agree beforehand what will both do, if it happens. If no such agreement exists, man has to accept his share of responsibility.