As a gay realtor, I think I can speak to this.
If you search "gay vermont real estate" you'll see that my Google AdWords ad comes up often. This is because I have chosen to add those keywords to my Google AdWords campaign.
However, if you visit my website
www.troyrichardson.com there is no mention of my being gay, or being gay-friendly. The reason isn't not in my meta-tags on my site is discussed below. I feel that being gay is part of my personal life, and doesn't need to be splashed all over my website- however, if someone is specifically searching for a gay or gay-friendly realtor, I want them to know that I am here.
It's marketing, just like any other type of marketing - think about those realtors who put Christian, Jewish or Korean as their keywords, because if people are searching for a christian or jewish realtor, those web sites will get picked up.
Claiming to be 'gay-friendly' is not illegal, it's advertising. As you said, what would be illegal is if you refused to work with gay clients in a state that had sexual preference anti-discrimination laws. This would be the equivalent of refusing to work with an african-american Customer, who are protected at a federal level.
As for the "lesbian-friendly" comment, I grew up in a time when the gay community wasn't so fragmented, and we used the word gay to describe people who were involved in same-sex relationships. However, certain groups over the years did not agree with this, and started wanting their own label, because they didn't feel included. Now we have LGBT, which is lesbian, gay, bi-sexual, transgendered as a moniker. Since I still feel that the term gay encompasses men and women involved in same-sex relationships, I don't feel the need to specifically say gay or lesbian, so you won't find my site listed under "lesbian vermont real estate" - I work with gay female customers and clients just as much as I work with gay males.
Website keywords are tough to manage, and just putting a keyword into your meta tags is not going to help you a lot. These days it's more about the words on the page and the links. A search engine isn't going to give much credit towards a meta keyword that isn't repeated in your text frequently, because the search engines have gotten smarter, and know what a bogus keyword is these days. This is another reason that I don't have gay as a meta-keyword - the search engines will practically ignore it because the word isn't repeated in my text.