It's my opinion that most of the people who argue against gay marriage use religion as a crutch because they wouldn't come off nearly so well in the polls if they said "I just don't trust gays with families." Naive, but honest.
I'm working on a writeup of the difference between the covenant of religious marriage and the civil union of marriage and how the Bible actually *does* come into play. :) The gist of it is basically that marriage in the Jewish and Christian faiths, at least, *is* something designed around raising a family and that the family and that family unit is actually core to the faith itself --- but there's nothing saying anything against a gay couple adopting. It was a reaction to a much higher mortality rate and the view that a lost chance for conception was a lost human being.
Now that we know better, it remains to have some people get over their prejudices and realize that from the faith-based perspective, the people who are perpetually childless are actually more out of keeping with the whole point of the church, which to provide moral teaching and build a community under the auspices of God. When a woman is already treated like property and a broodmare, the view that a marriage is a sacred trust and the raising of and providing for children is the most important task for a man and woman allows the woman to maintain a vital role in the family unit. (Not the best one, but vital nonetheless). It makes her indispensable for more than just birthing and prevents her abandonment or neglect... and it is then a role that is spelled out clearly, the community clearly accepting no alternative and finding a splintered family or a non-child-rearing family to be dangerous to their way of life. A religion that is centered around the continuation of and care for an existing family is actually a really *nice* thing, except for the fact that it doesn't really allow a whole lot of leniency in how the family comes about or whether everyone wants or needs to have children. Once child mortality was out of control, but that's no longer the case. The rules which were designed by a community to help perpetuate that community may be relaxed now.
So, really, the prejudice people are actually battling is the view that everyone should get married and start a family. When people started getting married and *not* intending to start families... perhaps that's when someone should have spoken up and said "well, we're either going to have to call that something else or relax our definition of "marriage" in the church, because the whole point is supposed to be that you're wedding for the production of the next generation."
The biblical warnings do make sense. They discouraged gay men and women from abandoning their existing families and encouraged them to help contribute to the population (at the cost of their personal freedom, but...). They warned of the dangers of promiscuity on the family unit. They helped to discourage actions which would undermine the trust between two people and interfere in the raising of children. They helped to make sure that everyone knew that anything preventing the proper care of wives, husbands and children was unacceptable. The penalties were harsh. I wouldn't run off to live with my girlfriend if I knew I could be stoned to death for leaving my children behind, forced though they were on me. But more importantly, from an early age it would have been made clear to me that leaving my husband or my children for my own selfish desires was sinful and selfish. The desire to be with a woman instead of a man wasn't sinful, but the desire to do so at the expense of my family was, as having and raising a loving and moral family was pretty much the one thing my community asked of me.
Any home which can provide a loving and nurturing atmosphere for children and want them should, by all means, go right ahead... and know that they're still within the spirit of all those verses the anti-gays wave around, gay or single or hetero or more than 2 ... the focus is and has always been on the production, growth and wellbeing of the next generation.
And people are pretty stupid when they hold up God to rationalize their prejudices... because that isn't the kind of god God is.