Adopting a child as single mother means that you refuse the child a father figure altough an adopted child is likely to still have many male role models. Not only do you risk denying a child the right to have a father you also remove them from a culture and most likely at least one birth parent.
I think society underestimates the influence and importance that a father has on a child. I don't doubt that same sex parents should be any less capable of raising a family. Gender does not determine suitability at least give an adopted child the security of having two parents instead of one.
Everything is relative and there is no perfect parent just as there are no perfect people. Anyone who claims the opposite is disillsioned.