Here's the problem I see with it, and to be fair this may significantly depend on what part of the world you're in (I'm in the US)-- the word "preference" usually has the connotation of, both options are acceptable but I like one more. So for example, if one says, "I would prefer to drink some orange juice rather than a bottle of poison," the statement might seem silly, sarcastic, or ridiculous-- because who really is okay with drinking a bottle of poison? "Preference" is a misleading term-- really, no one would drink the poison. Likewise, if I say, "I would prefer to date women," the implication is that you would settle for dating men. In the context of sexual orientation, this only really makes sense for a bisexual to say, because they are attracted to both sexes by definition. (One could argue semantics and pick out edge cases, e.g. "I'm straight but I would date someone of my same sex if I never had to have sex with them, and they gave me a shitton of money!" or whatever, but that's the general idea.)

That's just my take on it. The context I see TRAs use it in, at least, is: "Oh, lesbians would be fine with dicks, they just prefer to not date someone with a dick," because TRAs believe sexual orientation is a moral choice that one makes. When actually, pretty much all lesbians would not be comfortable with that (or only in the context of a sexless relationship, or whatever)...

Again, I'm sure this may differ by region. But I know that's what TRAs typically mean.