For point #2, we can follow the idea to its logical conclusion. (Note: I have zero issues with gay parents, this is just a thought exercise)
If it stands to reason that if the children of lesbian parents are indistinguishable from those of mixed-sex parents, then men play no particular role in the upbringing of children, no? They can be swapped out by a woman with no change for the child.
And conversely if a child raised by two gay men are indistinguishable from mix-sex parents, then woman play no particular role in the upbringing of children, no? Replace the mother with another man and nothing changes in that child's upbringing?
And therefore, one could argue that a single parent (only a man or woman) are only worse off due to limited parental resources (not to be underestimated), but not because an opposite sex partner would add anything unique to the childrearing.
Children raised in a non-dysfunctional two parent household (regardless of parental gender / sexual orientation) witness healthy interdependence, cooperation, negotiation, trust, respect, conflict resolution, love etc. between their primary caregivers during their critical development period. Raising a child with a good partner is about more than resource availability.
Having both genders is really valuable, by providing role modelling and interaction with the neuro-diversity of both genders.
Of course these days some people think the genders are not different, or whatever, some people think every strange though under the sun. I'm not speaking to those people they can believe whatever they want.
Have a read of 'Families and how survive them' for some detailed analysis of the roles of both genders in childrens development at various stages in their life.
If it stands to reason that if the children of lesbian parents are indistinguishable from those of mixed-sex parents, then men play no particular role in the upbringing of children, no? They can be swapped out by a woman with no change for the child.
And conversely if a child raised by two gay men are indistinguishable from mix-sex parents, then woman play no particular role in the upbringing of children, no? Replace the mother with another man and nothing changes in that child's upbringing?
And therefore, one could argue that a single parent (only a man or woman) are only worse off due to limited parental resources (not to be underestimated), but not because an opposite sex partner would add anything unique to the childrearing.