Transguys have a special place in my pansexual heart. However, it's not because of the genitals or body or whatever. It's because of their perspective. They already "get" my situation and I don't have to explain myself or my process to them. It's very easy for me to relate on a deep level to my boyfriend (who's a transguy). Obviously we have more in common than being trans (not a good basis for a relationship) but it's just an added thing (I was going to say bonus, but I don't wish being trans on anyone). And I have to say it's nice to not have to explain my boundaries in the bedroom, I just have to say what they are and he just KNOWS why.
If I had met him and he were a bioguy, transwoman, biogirl, however, I'd still date him/her. Sure I'd like to steal his genitals if he were biologically male, and I would have to explain more of what's going on in my head were he not a transguy, but I can't say his lack of transgender status would exclude him from my dating pool. He's kind-hearted, a nerd, and he makes me laugh a lot. That's what matters. Genitals, boobs, whatever, don't factor into the way I see a person's identity.
I'm not personally offended by your preferring transguys to bioguys, but I know MANY people that would be.
Once you become more comfortable with yourself, it might be easier for you to date a bioguy. I'm not saying it would for sure, but that seems to be how it goes for most people.