I have started a new thread as I do not want to derail the other David Quinn thread
The childs right in question is a child being raised by it's two biological parents. A homosexual familial unit cannot do this by design.
A child's rights extend far beyond "biological parents". They have a right to a stable, loving, supportive family. It is not true that any heterosexual familial unit is better than any homosexual familial unit at preserving these rights. This obvious fact is ignored by Quinn's argument. Also, you have said it is by design. That obviously isn't true at all. It is not the intention of a homosexual familial unit to not preserve a child's rights.
By formerly recognising homosexual familial units, the State opens the way for systematic adoption by gay couples, one of whom is the biological parent of the offspring being adopted by the other partner. If you think that you can enable marriage but draw a line at adoption (where one of the partners in the homosexual unit is the biological parent) then you are, I think, engaging in sophistry.
Ironically, your subtle switching of "adoption" to "adoption (where one of the partners in the homosexual unit is the biological parent)" is a classic example of sophistry. A couple, regardless of their relation to the child, should not be allowed to adopt if they are unfit. If Quinn wants to argue that gay couples are not fit to adopt, then he should do so. Saying gay marriage should be illegal because it opens the way for adoption is as useless as saying premarital sex should be illegal because it opens the way for single mothers.
In your first paragraph, you mentioned the right of a child to be raised by its biological parents. I find this right to be inconsequential compared to the right to a stable, supportive, encouraging household, but I should point out that by speaking out against adoption by biological parents with a same-sex partner, you are arguing against your earlier point.
Consider that the State-element of marriage confers rights and protections on a couple in the context of a unexpressed expectation that the male/female family unit will provide the State with the replacement population it needs. That some people won't be able to have children, that others will choose not to have children at all, doesn't alter the macro-intent behind the States support. So, the question isn't the intent of the couple getting married. It is the intent of the State in their providing the benefits that accrue to a married couple.
The above is not an argument against gay marriage, as they would be included in the group of married people who are unable to have kids. Why, in your opinion, do sterile couples get married? And why, in your opinion, does the state allow sterile couples to be married? I wholly reject any speculation about "unexpressed expectations". It is preferable to the state if its member are productive contributors to the well-being of society. Stable unions (i.e. Marriages) help achieve this.
