Im reminded about how other cultures traditionally delt with these issues.
Male Dominated cultures..we'll, we know that song and dance, women are property. Im glad we've moved away from that.
Female Dominated or Gender Neutral cultures, in the vast majority Fathers aren't concidered related to their children, its none of their business. The woman chose to have sex, its her kid.
I could definately see the later working, if a woman chooses to have unsafe sex..thats her perogative and her kid.
But asking women to share equal responsibility to men would never fly.
I always liked the option where either parent could put their half of parental rights and responsibilities up for adoption without the consent of the other. If you didn't want to be a father you could offer some childless man the rights (and responsibilities) of being the childs father, and the biological mother could deal with it since the babies right to a proper father who can support it outweighs her choice to have unsafe sex.