When I'm quite bored, or only a little, I'll wikipedia something on my magic phone for a quick read. "Engagement rings" was one of the more interesting topics. Customs vary so greatly from culture to culture and country to country, that I don't see why gay couples should feel that they have to follow any specific rules, or even have to stick to rings.
If I ever have the luck of being affianced, I doubt I would want a traditional diamond solitaire that the ladies get. Many same sex couples exchange simple bands in gold or platinum when they become engaged, and let the rings do double duty as wedding bands as well.
I'm not really a jewelry person, but I would like to receive something special in addition to a wedding band. The key pendants from Tiffany fit the bill for me, they run the gamut from sturdily masculine to undoubtedly feminine. I'm rather partial to simple styles done in yellow gold or black bone china.
I like that they aren't traditional, but have a romantic quality with "key to my heart" imagery. If you or your partner would like something sturdier, a watch would be a great option. I love vintage Omega Seamaster watches, especially this one from 1982.
Have a great week!