Ok, being a part of an 'affinity group' helps only if you're involved in that community. For example, even being asian, african america, native american, etc. is one thing. Having specific examples of how you're a leader in the community or examples of how you're committed to your community will help much more than simply being a member of that affinity group. After all, the top schools are looking for leaders. They're looking for people who are committed to their communities and those around them. They're looking for people who are active. If you're a leader in your affinity group - whether it be gay, minority, or an animal rights activist - then I'm sure it can only make your application look better, not worse.
That being said, simply saying 'I'm gay' probably won't help much. Plus, those admissions committees have pretty accurate gay-dars.