I could be thinking of a different discussion, but as I recall, the actual argument was that not being attracted to post-op trans women strictly because they're trans (wanting to have your own biological children with your partner not withstanding) does imply a least a degree of societal bias -- namely that they're still being perceived as men, or at least "less female" on some level -- even if that doesn't make someone a horrible person or mean that we should all force ourselves to date people we're not attracted to, being that sexual attraction is a complicated beast and near impossible to control (not to mention that dating someone you're not into is bound to end terribly for both you and the other person).