There have been studies of children brought up by homosexuals and they do not support to view that they are just as good as a heterosexual couple raising children.
Just a small note – quite a few studies have actually shown that homosexual parenting teams are just as good as heterosexual ones, which has led to the APA’s consensus view on the subject. However, none of those studies were very well-designed (because they couldn’t afford the needed money). The celebrated, controversial Regnerus study of this year showed the opposite conclusion, but it, too, suffered from statistically-induced challenges due to the extreme rarity of stable same-sex households in the 1990s.
The answers so far have been very good.
However, as User124 points out, there are foreseeable circumstances where having a child be adopted by a homosexual couple is the best option in a range of bad choices. To take an obvious example, if the choice is between letting a child starve to death and placing him in a same-sex household, the adoption is preferable. This does violence to the child, as the Vatican rightly observes, but not nearly as much violence as starvation!
Of course, in the context of the American adoption scene, that scenario is unusual, to say the least. There may be other, more common circumstances, where a same-sex adoption is preferable to the alternative evils, but this is a matter for prudential judgement in particular unfortunate cases. On the other hand, Zach Wahls, the subject of the speech User124 linked, was deliberately denied a father – this, needless to say, was a grave injustice, whether or not Mr. Wahls or User124 appreciates it.