Adopting a child as a 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 yet at least give an adopted child the security of having two parents instead of one.
Adopted children are the only children that legally could have only one parent or legal guardian. While I also believe that not all biological parents make good parents adopting a child as a single parent are not an easy task that one should take lightly. It's not like getting a new pet or houseplant it requires dedication, effort and time.
If anything was to happen to the single mother, who will become the legal guardian if there are no other parent? What if there are no relatives to the single mother that would agree to raise someone else adopted child. Would that not be selfish ?
I tend to applaud single parents that raises their own biological children because they have no choice.
Everything is relative and there is no perfect parent just as there are no perfect people. Anyone who claims the opposite is disillusioned.