Such injustice is the result of government involvement in marriage, rather than paternity. Similarly, we can expect injustice due to government involvement in marriage (the Defense of Marriage Act) rather than paternity when dealing with children. Injustice will be government treating some people more favorably than others in the tax code, giving tax breaks to working couples where one works and the other doesn't. And where government gives tax breaks to those who can and do have kids, while charging more for those who can't or don't.
Wouldn't it be better if government treated us all equally as individuals?