This is a good one to add to the list of things not to say on a first date. And if it does somehow come up, at least have enough sense not to follow it with "I also don't read biographies/memoirs because I consider them fiction."
/Don't ask
But then I have a fairly naive concept of a date as a (potential) meeting of minds, rather than a PR exercise.
It doesn't help that information is embedded structurally in a novel, which makes it harder to tease out. But good fiction (say, Ulysses or Crime and Punishment or Nine Stories) is as rich as any essay or monograph.
It's also not clear that density, in the strict sense, is what I'm after when I read. If so I'd be buried in technical papers, when what I really want is something to tickle my brain; to that end, I wouldn't want to lose any one of arXiv, the Engineering library, or the literature section at Borders.
I do read some biographies/memoirs though :P