I disagree with you there, Susan. A good argumentative paper always includes the other side's arguments, even--especially, actually--if you don't think they are valid. A strong thesis statement supporting your viewpoint is of course necessary, but so is including the other side's point of view--and why you think they're wrong.
For instance, one of the positives of allowing gay couples to adopt is the fact that there would be more adoptive parents in a world with too many children needing adoption. You could prove this point until you were blue in the face and still not have an argumentative paper, however, and it would do nothing to convince those who don't already agree with you.
Instead, you need to show the positives (like the one mentioned above) AND demonstrate that the negatives are either a)outweighed by the positives, or b)complete non-issues. The most common (and most rational-sounding) objection I've heard to gays adopting is that children need role models of both genders in order to develop properly. Do some research into this area, and find out if there's any scientific basis to it. If there is, you will need to explain why the benefits of letting gay people adopt outweigh this detriment. If there is NOT any scientific evidence to back up this claim, you will need to explain the lack of evidence, and the scientific findings that show it is not an issue.
Finding reasons not to let gays adopt is easy--just google a few search terms like "gay adoption" and you are sure to find many arguments. They might not seem reasonable to you, but your job in this type of paper is to show WHY these arguments are unreasonable, not simply to ignore them.