Let's deal with some of those questions shall we?
First of all, the statistics the right wing uses for that argument are generally statistics comparing married parents to single parents. Now, I don't know about you, but in my opinion raising a child on your own is a very hard job, and presumably that single parent is single due to loss of a partner which leaves a whole host of other issues ranging from death to divorce. So, yeah, being a child of a single parent is probably harder for most kids.
Of course, the right doesn't compare straight couples to gay COUPLES raising kids, because those data generally show that Gay families raise well adjusted kids, and children of gay parents are pretty much the same as kids from straight homes. Children of gay couples are no more likely to be in trouble, and no more likely to be gay. They do have one difference: they tend to be a little more tolerant.
So, there's no evidence that kids do better---or worse -- in straight households. SO that's a FAIL for their first argument.
The second question is more puzzling. Because the right acts as though there magically won't BE any kids being raised by gay parents if they just ban same sex marriage. This leads me to wonder; do they think that all those gay parents are going to suddenly become straight and marry heterosexuals? Or, do they think that we will somehow not have kids if we can't marry, and all the children of gay parents will suddenly disappear if they deny us marriage? Or, do they think if we marry we will go steal other people's children? I really wonder how they finish that thought, because it makes no sense.
The FACT is that we will continue to have and raise children whether or not we can marry legally, whether our natural children, or adopted. The LA TImes reported back in the fall that 25% of the gay families in California were raising children. Thus the only effect of preventing our marriage is to leave our children vulnerable and our families at risk. I think, really, that's what they want: they want our families to be at risk and disadvantaged for a "punishment" for the "sins of the parents". Rather sad, when you think about it. And that would be a FAIL for their second argument.
Update: Here's some great language from the Iowa court decision:
The court further noted that the County failed to show how the best interests of children of gay and lesbian parents, who are denied an environment supported by the benefits of marriage under the statute, are served by the ban, or how the ban benefits the interests of children of heterosexual parents. Thus, the court concluded a classification that limits civil marriage to opposite-sex couples is simply not substantially related to the objective of promoting the optimal environment to raise children. This conclusion suggests stereotype and prejudice, or some other unarticulated reason, could be present to explain the real objectives of the statute.
The American Academy of Pediatrics finds gay marriage has a positive effect on children.