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Basically, they know exactly what they are doing. They are using the word "preference" on purpose so that people will really start to believe that homosexual inclinations are actually a preference, and if it is a preference, then, of course it can be changed. You, a straight man, prefer vagina, however, if only you unpacked your genital fetish and unlearned the heterosexuality forced upon you by society, you would gleefully gobble up some delicious cock. The truth is that you only prefer vagina because you are a horrible person. A good, openminded person would prefer both. This is basically what L and G people are being told. Their orientation is bullshit, everything is a preference.

The fact of the matter is, even many bisexuals do not "prefer" trans people regardless of penis/vagina. Human beings are biologically wired to be sexually attracted to other human beings, and when we see something uncanny, like a person who has been treated with cross-sex hormones or had top or bottom surgery or other kind of surgery to look more like the opposite sex, unless that procedure was extremely good and convincing and natural looking, it is likely that our bodies will produce a "red flag" feeling and we will not be able to be attracted to this person. Just imagine an attractive (born female) woman, but she has these rock hard bowling ball -like breasts bolted onto her chest and her skin is stretched out and her nipples are pointing in two different directions and it just looks painful and uncomfortable for her. Objectively speaking, these are big round boobs so they should be attractive, but in reality, don't you get a "WTF? Oh girl! What did you do to yourself???" feeling instead?

Another thing is, especially for lesbians and bisexual women, a large man costumed as a woman produces a feeling of fear and unease, it's like being a sheep, and there is a wolf in sheep's clothing in your flock, and it's looking right at you, and everyone else is calling you an asshole for not accepting that this wolf is a real sheep. Now he might be the nicest wolf in the world who would never dream of hurting a sheep, but the sheep doesn't owe that wolf the emotional labor of giving the benefit of the doubt.