This has always been a subject that hits pretty close to home for me, since I know two people who are the adopted children of gay parents. Now, I'm fully aware of the fallacy that goes along with this (personal experience, individual examples, etc.), but I've never seen any evidence for psychological damage instilled by having two dads. In fact, quite the opposite. They're really some of the most tolerant and friendly people that I know. I could go on about them, but you get the idea.
I guess I'll address this at SG-1 mostly, just since he's had the most to say. No worries though, it's all on friendly terms.
I never really bought the whole issue of needing a male and female parent as a role model. It certainly doesn't jive with my own experience at least. Now, I don't doubt that a child needs exposure to the different aspects of different genders, probably from a young age. But does that need to be offered by the parents alone? Humanity has been an incredibly social species throughout it's history, and the "normal" family structure of children being raised by two parents has more often than not been abnormal. Today, unless the family is completely isolated for some reason, parents aren't the only one's raising the kid. From the very beginning, a person is shaped by their mother, father, close friends, aunts, uncles, media, and surrounding community. I don't think a person needs a female presence in the form of a mother for example when their lives are inevitably filled with females playing role model in everyday settings. Certainly not for psychological development anyway.
http://www.cpa.ca/cp...er 2006 (1).pdf
I think the idea that a child's psychological development could be damaged due to their "discomfort with having gay parents" is based on the fact that society in general is currently uncomfortable. If someone is raised by gay parents, would they have any reason to feel uncomfortable? Probably not, at least not through any natural disposition. There are certainly statistics out there that show people raised by same-sex couples are more prone to mental problems, but this is almost always attributed to the reaction of society, not to the home environment itself. Using relationship advice as an example, I'm not sure why having two dads would discourage someone from seeking it. If they really do love each other, a gay couple should have no trouble talking about the more general points of a relationship with their kid. I know there are differences between homosexual and heterosexual relationships, but if they're good parents, you would think that they would come to the conversation prepared for this.
And really, the fact that kids with gay parents are more frequently bullied isn't an argument against gay adoption. It's an argument against bullying and the toxic attitude our culture has towards gays. In any case, arguing that gay adoption should be discourage because the result is a less well-rounded individual seems like a slippery thing to say, since it brings up the tendencies of children in every minority and demographic. What can be said about the differences between children from white families and black families?
In relation to children living in orphanages, I would say living with gay adoptive parents would be quite preferable. From what I've read of child psychology, one of the most important aspects of early development is the presence of an emotionally attached, caring parent figure. In a group environment, particularly with orphanages, that aspect of childhood is often missed. Not as a result of direct abuse, but as a result of care-givers being too busy to create that emotional connection. In most research, the focus is almost never on the gender roles that parents can have. The focus is on the emotional relationship they have with their children. Put simply, it's not the gender of the parent that matters, it's the quality of the parenting. I know it's cliche, but love really is the most important thing.
All in all, I don't see any reason (psychological, societal, religious, etc.) why same-sex adoption, let along marriage, should be discouraged.