IMHO, its not about who is better at parenting. Its about defining parents based on their sex needs and telling that to the child.
If you allow homo-sexual marriage and homo-sexual child bearing somehow you have to explain that to the children when they realise they have two dads or two mums or a friend has two dads or two mums.
So is the answer sex education in kindergarten? Most of us believe children should not be concerned with sex needs at such an early age. Is it worth throwing away our morals because of the deviant needs of a few?
I have no problem where a child has one dad or one mum and dad or mum has a friend who helps. But defining marriage as hetero or homo and putting that label on every child in our society cannot be good for anyone.