I would say that for me, it is a mixture of how my parents are, and the kind of people I grew up around, including any ethnic details thereof. It also reflects positive feedback in terms of who I was successful with -- after a while, I realized I did well with X type of person and that my relative value was higher with women of color, so I go where the market is.
Example --
My parents are both kind of nerdy and eccentric -- anyone that I was going to really be happy with long term (and not just superficially infatuated with) was going to have to share some of my nerdy and eccentric interests and qualities. In other words, someone who is ok with going outside of the canned, mediocre content we are all sold every day.
Ethnicity --
When I was growing up, I was initially attracted to what everyone else seems to be attracted to, white girls with blonde hair. Some of this is from conditioning - our media constantly pushes a certain image to the rest of the world in terms of what is supposed to be desirable in a woman. But this is just part of it -- some of it is just that blonde happens to be considered universally attractive (there is evidence women have been bleaching their hair since the ancient Persian empire), and some of it is genetic attraction -- I'm tall and blond, so it would make sense that I would be attracted to someone who looks like me.
But -- that wasn't how things went. Instead, I found that a lot of women of color were attracted to me, and wanted me more than their white counterparts. Lots of reasons for this, which we can cover separately -- but basically, I went where the market was, and my success there shaped my tastes in almost a Pavlovian way. So, because of this, all of the women I have dated have looked almost identical -- black hair, asian-looking eyes, shorter stature.