Do you guys judge people by their past Significant Others?

[James]

I kind of have a habit of judging girls I meet by who they used to date, if I know. I have a pretty high opinion of myself and if I find out the guy they dated for a significant length of time was not up to standard, I start wondering if I want to be in that same category. Cause it does kind of group you together. And if she'll date that kind of guy it doesn't bode well for her appreciation of something better.

I have been told by girls that this isn't really fair, although they never really explained why very well. . .

Anyone share this quirk to some degree or can tell me why this would be a bad thing?