The problem is, we get into "should" and "shouldn't" and base our arguments on stereotypes. "The world is a bad place, and children will get discriminated against." Unfortunately, the stereotype here is of the world in general, not of gays.

They should have the right to marry and to raise kids. Whether or not a specific couple should is not an issue for people who are discussing a specific couple. Rights aren't granted to specific individuals, they're granted to everybody.

As to protecting children...
Some communities would be better than others for a child of gay parents to grow up in. Some people are better than others at being nice. If certain people are denied the same rights as others based on solely on potential (or even probable) misbehavior by people outside that group, it's just passive discrimination. It's punishing the victim for what they might be victimized for.

Parents usually want to protect their kids. People who care about them want to protect them. We don't live in a perfect world, though. Kids have to learn how to walk, even though they may fall. Kids have to learn how to deal with bullies and potential kidnappers, molesters, peers who do drugs or get into trouble...any number of things. Kids have to learn to take a stand for what they believe in. Some will be hurt in the process, others will go on to do great things, and I think the difference, many times, can be how valid their stand is in the eyes of society. If society makes a law saying "You are in the right," it's a start in helping a kid deal with bigots.

The rights come first.