While not strictly part of the futanari genre, Pururin does have a section devoted to yaoi porn that I think a lot of you are going to love. I mean, there's a mixture of content here anyway and things aren't tagged so well. With that in mind, if you're the type of person that desires to see hentai characters engaging in weird, wonderful and bizarre sex, I can only recommend this as being a top destination to do exactly that.
Pururin has advanced search filters, the ability for you to sort by popularity, rating and view counts as well as attractive thumbnail displays that are sure to put you in control of the material you enjoy with very little effort. Yaoi's big here too: there are hundreds of results that fit this category.
Note that almost all of the material here is hosted in Japanese. If you can read it, then go crazy - but if you're an English user, you might have to get used to looking at the pictures only and guessing what is being said.