I definitely think a parts preference is valid.

That being said, I would rather not make a blanket statement about how I wouldn't date a particular type of adult human, purely based on the type of people I have met up to now.

I find individuals attractive. In the past I thought "oh my type is long haired Indian men" but it turns out I'm attracted to all sorts of people with all sorts of bodies, depending on the people I actually meet and their energy.

So while you can say "I've never been attracted to a trans man before" I think it's possible there might be at least 1 in the whole world that you would find attractive.