It's been my observation from my own experiences, both thinking back to when I was a child and now as a parent, that men (fathers) and women (mothers) both bring something different and unique to the family and parenting experience. That leads to a more rounded upbringing for children. That's the traditional family unit (a mother, a father, one or more children). I don't believe that homosexual couples (two fathers or two mothers), in general, can bring that same kind of upbringing to a child.
That's not to say that they can't be good parents in their own right. But I believe that a child up for adoption, in the long run, would be better served in a heterosexual parent family than in a homosexual parent family, so that they can get a more well rounded upbringing.
Again, I don't disagree with any of this. I already conceded that an opposite-sex partnership is preferable for child-rearing. But the reality is that no matter how you try to spin it, it really isn't an option of same-sex parents vs. opposite-sex parents. It's a question of same-sex parents vs. no parents. Furthermore, you still haven't responded to why you don't have an issue with single-parents but do have a problem with same-sex parents. Single parents also provide only one side of the gender spectrum, but they also provide only one breadwinner, only one person to talk to, only one person to help nurture the child. I think it's pretty clear that if you're not worried about specific HARMS of being raised by same-sex parents, a same-sex couple is preferable to a single parent.