Common sense, people. There is no such thing as gay people in the Japanese entertainment industry. If there were, they’d be beaten to death with a stick and have their careers driven down the deep, dark hole known as oblivion. This is after all, a country that crucifies their entertainers for making jokes about women’s amnion fluid going rotten after the age of 35, and where famous stars have to apologize for their rotten son’s or brother’s bad behaviour.
There are no yuri seiyuu. They only want you to believe they are so, in the interest of getting your male/female hormones all overworked so that you buy their photobooks and merchandise in order to fantasize some more.
And of course, over-excited fans like me lap it all up. After all, it’s one of the fun and more light-hearted aspects that takes your mind off the fact that the seiyuuu business is a depressing place, rotten to the core. (underpaid, underappreciated, blablabla). But hey, it’s a really incestuous industry anyway, where every seems to be the friend of someone who knows somebody else.
Out of boredom, I drew a quick chart (click for larger version) to illustrate some of the relationships going on in the seiyuu world, seen from the eyes of a fan wearing thick yuri goggles. You can play spot the seiyuu at the same time too, since I neglected to put their names in (oops).
EDIT: Uploaded a labeled version here.

Red: Yuri! Confirmed yuriyuriyuri!
Blue: ‘Good’ friends
Orange: Yuri(?)
Green: One-sided love
Yellow: Deep admiration
Please don’t harp on about how inaccurate it is - the colour of the arrows is subjective
Please don’t tell me Ai & Mai should have a red arrow, I think they are about as yuri as the Powerpuff Girls.
Sorry if you were expecting some deep essay on the evolution of yuri female seiyuu, I don’t have the will to write that sort of thing :/