But it does imply ranking, i.e., women 1, men 2. When someone who is monosexual, only 1 sex in their ranking, the other option is never considered as a potential partner in any way.

People usually don't say the second part of the sentence. No one is going to outright say 'I prefer women. I hate men.' or the alternate. The title lesbian/gay/straight should be enough to imply it. However, since there are more bisexual lesbians, political lesbians, and queer people around these days, they can't fathom the idea of a woman (or man) only being attracted to a single sex. When they talk about sexual/genital preference, it's well known that many people in the modern 'queer' community (LGBTQ) think people are sexually fluid and can change their 'preferences' if they think about them enough.

I do, however, understand that people don't use preference properly, but I'll also add that 'sexual preferences' and the term 'preferences' was often used by conservatives and the religious to imply that homosexuality and bisexuality are an active choice to love the same sex, not an innate characteristic.

Sexual orientation doesn't leave that ambiguity. It's clear cut and doesn't undermine what LGB activists have been working towards for decades.