how many bisexuals need members of the same sex?

My wife is bisexual and she has told me that she is more attracted to women in general and that she just gets in moods where she just has to have a woman in bed. Here lately she has been looking up a lot of lesbian porn on the internet and I am starting to believe that she wasn't kidding when she said she needs a woman from time to time. So basically is it just a bi thing to be more attracted to the same gender more than the opposite gender. or is that just my wife as an individual or is it a rule of thumb that bisexuals need to just get it on with the same sex to survive