Prop 8 is about marriage, not parenthood. Prop 8 seeks to reiterate for CA government administrators that there is one and only one set of proper recipients of a marriage license: a man and a woman as husband and wife.
One does not have to be married to be a parent, and I would imagine a gay couple could make just as good a parents as a straight couple.
Your question stimulates a new thought for me, though.
Does a straight couple who are parents make better parents for straight kids than gay kids .. and do a gay couple who are parents make better parents for gay kids than straight kids?
Much about good parenting is being an effective good role model of adult romantic behavior.
When it comes to such role modeling, that's about a whole lot more than just the form of going through the motions -- it's about the substance of such, and that includes who and what the parents are.
A straight couple can model well romantically their roles, but their gay kid may end up a bit substantively confused, as by both physiological and psychological nature, there are significant substantive differences exemplified in romantic relating and roles in straight couples compared to either type of gay couples.
Not that there is anything that can or should be "done" in recognition of this .. but the thought just struck me and I decided to post it.