If we tolerate your neighbour doing something, then by all rights we should tolerate you doing that same thing too.
Try not to read more into what people say than is actually written.
Source?
We allow heterosexual couples to marry, even though there exists a risk for infection and violence, so by all rights we should allow homosexual couples to marry, despite the same risk.
What problems, precisely?
Actually, there have been four decades of studies, the unanimous conclusion of which is that same-sex couples are no better or worse than opposite-sex couples at child-rearing. Indeed, children raised by female-female couple have a slight statistical
advantage.
Moreover,
the APA states "there is
no scientific evidence that parenting effectiveness is related to parental sexual orientation: lesbian and gay parents are as likely as heterosexual parents to provide supportive and healthy environments for their children
research has shown that the adjustment, development, and psychological well-being of children is unrelated to parental sexual orientation and that the children of lesbian and gay parents are as likely as those of heterosexual parents to flourish
.".
The Canadian Department of Justice concluded: "The strongest conclusion that can be drawn from the empirical literature is that the vast majority of studies show that
children living with two mothers and children living with a mother and father have the same levels of social competence. A few studies suggest that children with two lesbian mothers may have marginally
better social competence than children in traditional nuclear families, even fewer studies show the opposite, and
most studies fail to find any differences. The very limited body of research on children with two gay fathers supports this same conclusion."
So... who's experimenting? People have been raising children outside of the traditional nuclear '1 mother + 1 father + 2.3 children' family since families first began.