Do I have evidence? No.
Do you have any evidence that any other form of parenting is just as good?
That's not just a good rule of thumb, its a Constitutional rule in the US (though the standard to meet varies depending on the kind of discrimination from a very low standard—the rational basis test—to a very high standard—strict scrutiny.)
That's quite a stretch honestly.
Whether it's primarily by gender composition or by biological relationship, what you are proposing is strictly additional discrimination over the alternative you asked to be justified of supporting two-parent families generally and without discrimination among them.
As such, you should bear the burden of supporting that additional discrimination as desirable, rather than your interlocutor bearing the burden of justifying its absence as desirable.
Have a good one.
Or do you refuse to post your evidence until other people post theirs, like some kind of standoff?
You have definitely read something into it that was not actually stated