Does a kid's right to be parented by biological mom and dad trump the right of sexual expression?

You raise some interesting points what opponents of gay marriage are actually worried about (whether or not those fears are reasonable is another issue).

But the above point in particular strikes at much of the basic disagreement here. I do not believe anyone has the "right" per se to be parented by a biological mom and dad in a nuclear family unit... in that it's an entirely unenforceable right if life circumstances do not allow for it.

One or both biological parents could die. One or both biological parents could be flat out disinterested or otherwise mentally or emotionally ill-suited for raising their own children. You can crow all you like about the rights of their children in that case, but your dismay will not provide them two fully functional biological parents. In short... we all have to play the cards we're dealt when we're born... socially ideal circumstances or no.

Encoding the married mom/dad family unit into law as the only "legitimate" way to form a family automatically excludes people who cannot or will not grow up in such an environment. Does it harm or help children to insist that they live in an inferior "sham" of a "real" family when they don't have a mom and a dad at home?

There is no way to stop people from having children outside of the proposed "ideal." It has always happened and always will happen. Unmarried women will continue to get pregnant. Gay people will pop out kids inspite of being gay. Unless you also propose forced sterilization of people who intend to reproduce outside of the married heterosexual paradigm, there's nothing you can do to stop any of this from happening.

Gay civil rights issues address the reality of people's lives, the hardships and injustices faced by people who already do exist. "Traditional" activists concentrate on their as of yet unsubstantiated concerns of what horrible things might happen one day if....dot dot dot.

In my mind the laws are here to serve the ENTIRE population in the here and now... Gay people and their children are already here and living amongst us, and more are on the way. They deserve and require the full support of the society they participate in. We all have to share this planet. We all have to pay taxes and share public space and institutions, which means having to tolerate people who live and act and breed in ways you don't like. The "right" to legislate what other people can do with their bodies and what kinds of families they can have is NOT a right if there is nowhere else for the people who disagree with you to go.