I understand your point, to be sure; the main issue is that it so frequently comes with the reasoning of "ick, I don't want to get any transsexuals on me," so we're leery of it.
I really don't think where people want or don't want their genitals is anyone's business but their own. I've come to realize is not a popular opinion, but I still disagree with people getting all judgmental about a guy having sex with his sister, or about a guy not wanting to have sex or date a certain group of (usually downtrodden) people. Sure, maybe that person has issues. Maybe they don't. Either way, it's the same: No one gets to say to others who they should or shouldn't date, much less who they are intimate with.
I don't have an overly complex moral system. I work by a simple rule. Everyone's free to do what they want as long as it is not in detriment of someone else. This means that, yes, trans people should be allowed to be themselves and treated with respect. But it also means the same applies to those who for one reason or the other don't want to date trans people. You're certainly free to say people like that are wrong, and I would encourage you to make your case... with a minimum of insults and condescension if you want to be listened to.