I think that people should actually write the rest of their essay first, because they will then have a better idea of exactly what they want to say in their introduction/thesis statement. It helps to know what your arguments will be before you write your thesis.
From what you tell us, I think that you should say something like this:
Although many people are opposed to gay people adopting children, there is no rational basis for preventing same sex couples from adopting. The main focus of any adoption law should be the good of the child who is being adopted. Children will be best served if they are raised by suitable parents of whatever sex. Therefore, laws should make it possible for couples of whatever sex to adopt if they will make good parents.
I think that this would do a pretty good job of introducing what I assume you are going to be saying in your essay.