I recently spoke to wedding professionals in DC and one of the points I made is that gay couples don't like to be tokenized. Despite your best intentions, it's annoying for us to hear things like, "I went to college with a lot of gay people." And if you are a wedding vendor, there is no need to put rainbows on your website unless you do so in a clever way like in the photo to the left. While the rainbow is universally and globally known, there are other ways to show your inclusivity.
Truthfully, modern gays and lesbians aren't that into rainbows. They have a lovely meaning and the meaning of the pink triangle is very powerful - but we find both symbols to be cheesy and dated. Rainbows and pink triangles aren't going to offend gays and lesbians, nor are they politically incorrect. But they are very "old school." So while you can put rainbows in your marketing materials, you might find that the modern engaged gay or lesbian couple isn't into it.
If you choose to use a symbol, a more modern alternative is the HRC symbol, the yellow equals on the blue background. You may see this on bumper stickers - it's very common and we all know what it means.
Do you use rainbows, pink triangles or other symbols to signify your support of LGBT individuals?