If you think about it it's fairly obvious. If a child should be brought up by its natural parents then it doesn't make sense to introduce other partners into the family.
Firstly, if you accept gay people's relationships then you get fewer children and possibly fewer broken relationships. Secondly, the massively overwhelming number of children with step-families come from heterosexual parents, and there's no evidence whatsoever to support the notion that children brought up in gay households suffer in any way because of the orientation of their parents.
A public commitment between the natural parents ensures that a child is brought up by them.
As a parent of two IVF donor egg children, I can reliably say that there's absolutely no reason whatsoever that not having a 'biological' link to your children needs to in any way diminish your commitment, love or devotion to them. 'Natural' parents who do nothing more than provide a gamete and then years of neglect or abuse are a far, far worse threat to a child's wellbeing than a step-parent, regardless of their sexual orientation.
That's what I see as the role of marriage.
With the exception of the irrelevance of 'natural', there's no reason a gay marriage can't do exactly that. For the individuals concerned, probably far more effectively than if they are confined to relationships they don't really feel committed to because of social or legal constrictions on them being genuine about who they are.
And I don't agree with the "marriage is not about kids" view.
Which is fine, for you. Any marriage you choose can be about kids if you'd like. You don't get to tell everyone else what their marriage has to be about, though.
And of course other factors threaten family values, such as divorce.
Which is significantly more likely if you corral gay people into straight marriages as they only way they, who might be just as interested in raising children, can achieve their life goals.
And I don't agree that killing people is the way to prevent liberalism.
OK.
What do you mean by "Do these religious nutjobs think that homosexuality is so irresistibly attractive that everybody is suddenly going to switch camps given the chance? "?
I think that's probably a reference to the thick vein of panic in some religiously conservative communities - particularly in the US, that I'm aware of - that you can somehow 'train' somebody to be gay, or perhaps the reverse that if you never expose children to the idea of homosexuality they'll never have those inclinations, that being gay is a 'phase' or a 'rebellion'.
It's the 'you're straight really, you're just pretending because it's cool' parallel to the religious argument that 'you believe really, you're just pretending to be atheist because it's cool'.
O.