the claim of "best" isn't supported by research.
Research has shown that children do best with their biological parents - or at least with two parents of opposite sex.
Modern researchers have tried to overturn this idea with case studies of very limited numbers of children being raised by homosexual couples.
If you read those studies, you'll find ideas like this:
1.) Children raised by homosexual couples are not more likely to be abused by their parents than children raised by heterosexual couples, THEREFORE the gender of the parents is immaterial
2.) Children raised by homosexual couples have their sexual identities affected by their parents only slightly more than children raised by heterosexual couples, THEREFORE the gender of the parents is immaterial.
3.) Children raised by homosexual couples tend to do as well in school as children raised by heterosexual couples, THEREFORE the gender of the parents is immaterial.
Research like this abandons sound reason, clamoring for any evidence to claim that gender is immaterial.
A child raised in an orphanage might be trained to do really well in school, but that doesn't mean the orphanage is just as good of a child rearing environment as a heterosexual household.
Just because children raised in homosexual families tend to have similar trends on the extreme highs and lows of childhood experiences as those raised in heterosexual families does NOT mean that parental gender is immaterial.
There is research that shows children raised in homosexual families do suffer in certain categories as compared to children raised in heterosexual families - and just because you show that a child raised in a homosexual family doesn't get in trouble at school more than a child raised in a heterosexual family, doesn't undo the previous research.
Having good and loving parents matter. their genders are immaterial
The president got attacked by liberal scientists for suggesting that fathers were an essential and important determinant in raising healthy children.
Ideas have consequences - especially the idea that gender is irrelevant. It's not surprising that people think they can cut off their genitals to become a woman.